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AIRPLANE PHOTOGRAPHY


[Illustration]


AIRPLANE PHOTOGRAPHY

by

HERBERT E. IVES

Major, Aviation Section, Signal Officers Reserve Corps, United States
Army; Lately Officer in Charge of Experimental Department, Photographic
Branch, Air Service

208 ILLUSTRATIONS







[Illustration]

Philadelphia and London
J. B. Lippincott Company

Copyright, 1920, by J. B. Lippincott Company

Printed by J. B. Lippincott Company
at the Washington Square Press
Philadelphia, U. S. A.




                               TO MY WIFE

    A HELPFUL CRITIC, EVEN THOUGH SHE NEITHER PHOTOGRAPHS NOR FLIES




                                PREFACE


Airplane photography had its birth, and passed through a period of
feverish development, in the Great War. Probably to many minds it
figures as a purely military activity. Such need not be the case, for
the application of aerial photography to mapping and other peace-time
problems promises soon to quite overshadow its military origin. It has
therefore been the writer's endeavor to treat the subject as far as
possible as a problem of scientific photography, emphasizing those
general principles which will apply no matter what may be the purpose of
making photographs from the air. It is of course inevitable that whoever
at the present time attempts a treatise on this newest kind of
photography must draw much of his material from war-time experience. If,
for this reason, the problems and illustrations of this book are
predominantly military, it may be remembered that the demands of war are
far more severe than those of peace; and hence the presumption is that
an account of how photography has been made successful in the military
plane will serve as an excellent guide to meeting the peace-time
problems of the near future.

It is assumed that the reader is already fairly conversant with ordinary
photography. Considerable space has indeed been devoted to a discussion
of the fundamentals of photography, and to scientific methods of study,
test, and specification. This has been done because aerial photography
strains to the utmost the capacity of the photographic process, and it
is necessary that the most advanced methods be understood by those who
would secure the best results or contribute to future progress. No
pretence is made that the book is an aerial photographic encyclopædia;
it is not a manual of instructions; nor is its appeal so popular as it
would be were the majority of the illustrations striking aerial
photographs of war subjects. It is hoped that the middle course steered
has produced a volume which will be informative and inspirational to
those who are seriously interested either in the practice of aerial
photography or in its development.

The writer is deeply in debt to many people, whose assistance of one
sort or another has made this book possible. First of all should be
mentioned those officers of the English, French and Italian armies
through whose courtesy it is that he can speak at first hand of the
photographic practices in these armies at the front. It is due to
Lieutenant Colonel R. A. Millikan that the experimental work of which
the writer has had charge was initiated in the United States Air
Service. To him and to Major C. E. Mendenhall, under whom the work was
organized in the Science and Research Division of the Signal Corps, are
owing the writer's thanks for the opportunities and support given by
them. A similar acknowledgment is made to Lieutenant Colonel J. S.
Sullivan, Chief of the Photographic Branch of the Army Air Service, for
his interest and encouragement in the compilation of this work, and for
the permission accorded to use the air service photographs and drawings
which form the majority of the illustrations.

The greatest debt of all, however, is to those officers who have formed
the staff of the Experimental Department. To mention them by name:
Captain C. A. Proctor, who was charged with our foreign liaison, and who
acted as deputy chief during the writer's absence overseas; Captain A.
K. Chapman, in charge of the work on optical parts, and later chief of
our Rochester Branch; Captain E. F. Kingsbury, who had immediate charge
of camera development; Lieutenant J. B. Brinsmade and Mr. R. P.
Wentworth, who handled the experimental work on camera mountings and
installation; Lieutenant A. H. Nietz, in charge of the Langley Field
Laboratory of the Experimental Department; Mr. R. B. Wilsey and
Lieutenant J. M. Hammond, who, with Lieutenant Nietz, carried on the
experimental work on sensitized materials. A large part of what is new
and what is ascribed in the following chapters to “The American Air
Service” is the work of this group of men. Were individual references
made, in place of this general and inclusive one, their names would
thickly sprinkle these pages. It has been a rare privilege to have
associates so able, enthusiastic, and loyal.

                                                              THE AUTHOR

 NOVEMBER, 1919




                                CONTENTS


                             I. INTRODUCTORY

 CHAPTER                                                            PAGE

  1. GENERAL SURVEY                                                   15

  2. THE AIRPLANE CONSIDERED AS A CAMERA PLATFORM                     20


                         II. THE AIRPLANE CAMERA

  3. THE CAMERA—GENERAL CONSIDERATIONS                                39

  4. LENSES FOR AERIAL PHOTOGRAPHY                                    44

  5. THE SHUTTER                                                      68

  6. PLATE-HOLDERS AND MAGAZINES                                      87

  7. HAND-HELD CAMERAS FOR AERIAL WORK                                95

  8. NON-AUTOMATIC AERIAL PLATE CAMERAS                              102

  9. SEMI-AUTOMATIC AERIAL PLATE CAMERAS                             116

 10. AUTOMATIC AERIAL PLATE CAMERAS                                  124

 11. AERIAL FILM CAMERAS                                             130

 12. MOTIVE POWER FOR AERIAL CAMERAS                                 145

 13. CAMERA AUXILIARIES                                              163


        III. THE SUSPENSION AND INSTALLATION OF AIRPLANE CAMERAS

 14. THEORY AND EXPERIMENTAL STUDY OF METHODS OF CAMERA SUSPENSION   179

 15. PRACTICAL CAMERA MOUNTINGS                                      193

 16. INSTALLATION OF CAMERAS AND MOUNTINGS IN PLANES                 208


                 IV. SENSITIZED MATERIALS AND CHEMICALS

 17. THE DISTRIBUTION OF LIGHT, SHADE AND COLOR IN THE AERIAL VIEW   221

 18. CHARACTERISTICS OF PHOTOGRAPHIC EMULSIONS                       227

 19. FILTERS                                                         239

 20. EXPOSURE OF AERIAL NEGATIVES                                    247

 21. PRINTING MEDIA                                                  252

 22. PHOTOGRAPHIC CHEMICALS                                          257


             V. METHODS OF HANDLING PLATES, FILMS AND PAPERS

 23. THE DEVELOPING AND DRYING OF PLATES AND FILMS                   267

 24. PRINTING AND ENLARGING                                          279


                     VI. PRACTICAL PROBLEMS AND DATA

 25. SPOTTING                                                        291

 26. MAP MAKING                                                      304

 27. OBLIQUE AERIAL PHOTOGRAPHY                                      320

 28. STEREOSCOPIC AERIAL PHOTOGRAPHY                                 329

 29. THE INTERPRETATION OF AERIAL PHOTOGRAPHS                        351

 30. NAVAL AERIAL PHOTOGRAPHY                                        368


                  VII. THE FUTURE OF AERIAL PHOTOGRAPHY

 31. FUTURE DEVELOPMENTS IN APPARATUS AND METHODS                    383

 32. TECHNICAL AND PICTORIAL USES                                    388

 33. EXPLORATION AND MAPPING                                         401




                                   I
                              INTRODUCTORY




                          AIRPLANE PHOTOGRAPHY




                               CHAPTER I
                             GENERAL SURVEY


=Aerial Photography from Balloons and Kites.=—Photography from the air
had been developed and used to a limited extent before the Great War,
but with very few exceptions the work was done from kites, from
balloons, and from dirigibles. Aerial photographs of European cities had
figured to a small extent in the illustration of guidebooks, and some
aerial photographic maps of cities had been made, notably by the Italian
dirigible balloon service. Kites had been employed with success to carry
cameras for photographing such objects as active volcanoes, whose
phenomena could be observed with unique advantage from the air, and
whose location was usually far from balloon or dirigible facilities.

As a result of this pre-war work we had achieved some knowledge of real
scientific value as to photographic conditions from the air. Notable
among these discoveries was the existence of a veil of haze over the
landscape when seen from high altitudes, and the consequent need for
sensitive emulsions of considerable contrast, and for color-sensitive
plates to be used with color filters.

The development of aerial photography would probably however have
advanced but little had it depended merely on the balloon or the kite.
As camera carriers their limitations are serious. The kite and the
captive balloon cannot navigate from place to place in such a way as to
permit the rapid or continuous photography of extended areas. The kite
suffers because the camera it supports must be manipulated either from
the ground or else by some elaborate mechanism, both for pointing and
for handling the exposing and plate changing devices. The free balloon
is at the mercy of the winds both as to its direction and its speed of
travel. The dirigible balloon, as it now exists after its development
during the war, is, it is true, not subject to the shortcomings just
mentioned. Indeed, in many ways it is perhaps superior to the airplane
for photographic purposes, since it affords more space for camera and
accessories, and is freer from vibration. It is capable also of much
slower motion, and can travel with less danger over forests and
inaccessible areas where engine failure would force a plane down to
probable disaster. But the smaller types as at present built are not
designed to fly so high as the airplane, and the dirigibles, both large
and small, are far more expensive in space and maintenance than the
plane. For this one reason especially they are not likely to be the most
used camera carriers of the aerial photographer of the future. Inasmuch
as the photographic problems of the plane are more difficult than those
of the dirigible and at the same time broader, the subject matter of
this book applies with equal force to photographic procedure for
dirigibles.

=Development of Airplane Photography in the Great War.=—The airplane has
totally changed the nature of warfare. It has almost eliminated the
element of surprise, by rendering impossible that secrecy which formerly
protected the accumulation of stores, or the gathering of forces for the
attack, a flanking movement or a “strategic retreat.” To the side having
command of the air the plans and activities of the enemy are an open
book. It is true that more is heard of combats between planes than of
the routine task of collecting information, and the public mind is more
apt to be impressed by the fighting and bombing aspects of aerial
warfare. Nevertheless, the fact remains that the chief use of the
airplane in war is _reconnaissance_. The airplane is “the eye of the
army.”

In the early days of the war, observation was visual. It was the task of
the observer in the plane to sketch the outlines of trenches, to count
the vehicles in a transport train, to estimate the numbers of marching
men, to record the guns in an artillery emplacement and to form an idea
of their size. But the capacity of the eye for including and studying
all the objects in a large area, particularly when moving at high speed,
was soon found to be quite too small to properly utilize the time and
opportunities available in the air. Moreover, the constant watching of
the sky for the “Hun in the sun” distracted the observer time and time
again from attention to the earth below. Very early in the war,
therefore, men's minds turned to photography. The all-seeing and
recording eye of the camera took the place of the observer in every kind
of work except artillery fire control and similar problems which require
immediate communication between plane and earth.

The volume of work done by the photographic sections of the military air
service steadily increased until toward the end of the war it became
truly enormous. The aerial negatives made per month in the British
service alone mounted into the scores of thousands, and the prints
distributed in the same period numbered in the neighborhood of a
million. The task of interpreting aerial photographs became a highly
specialized study. An entirely new activity—that of making photographic
mosaic maps and of maintaining them correct from day to day—usurped
first place among topographic problems. By the close of the war scarcely
a single military operation was undertaken without the preliminary of
aerial photographic information. Photography was depended on to discover
the objectives for artillery and bombing, and to record the results of
the subsequent “shoots” and bomb explosions. The exact configurations of
front, second, third line and communicating trenches, the machine gun
and mortar positions, the “pill boxes,” the organized shell holes, the
listening posts, and the barbed wire, were all revealed, studied and
attacked entirely on the evidence of the airplane camera. Toward the end
of the war important troop movements were possible only under the cover
of darkness, while the development of high intensity flashlights
threatened to expose even these to pitiless publicity.

=Limitations to Airplane Photography Set by War Conditions.=—The ability
of the pilot to take the modern high-powered plane over any chosen point
at any desired altitude in almost any condition of wind or weather gives
to the plane an essential advantage over the photographic kites and
balloons of pre-war days. There are, however, certain disadvantages in
the use of the plane which must be overcome in the design of the
photographic apparatus and in the method of its use. Some few of these
disadvantages are inherent in the plane itself; for instance, the
necessity for high speed in order to remain in the air, and the
vibration due to the constantly running engine. Others are peculiar to
war conditions, and their elimination in planes for peace-time
photography will give great opportunities for the development of aerial
photography as a science.

Chief among the war-time limitations is that of economy of space and
weight. A war plane must carry a certain equipment of guns,
radio-telegraphic apparatus and other instruments, all of which must be
readily accessible. Many planes have duplicate controls in the rear
cockpit to enable the observer to bring the plane to earth in case of
accident to the pilot. Armament and controls demand space which must be
subtracted from quarters already cramped, so that in most designs of
planes the photographic outfit must be accommodated in locations and
spaces wretchedly inadequate for it. Economy in weight is pushed to the
last extreme, for every ounce saved means increased ceiling and radius
of action, a greater bombing load, more ammunition, or fuel for a longer
flight. Hence comes the constant pressure to limit the weight of
photographic and other apparatus, even though the tasks required of the
camera constantly call for larger rather than smaller equipment.

To another military necessity is due in great measure the forced
development of aerial photographic apparatus in the direction of
automatic operation. The practice of entrusting the actual taking of the
pictures to observers with no photographic knowledge, whose function was
merely to “press the button” at the proper time, necessitated cameras as
simple in operation as possible. The multiplicity of tasks assigned to
the observer, and in particular the ever vital one of watching for enemy
aircraft, made the development of largely or wholly automatic cameras
the war-time ideal of all aerial photographic services. Whether the
freeing of the observer from other tasks will relegate the necessarily
complex and expensive automatic camera to strictly military use remains
to be seen.




                               CHAPTER II
              THE AIRPLANE CONSIDERED AS A CAMERA PLATFORM


An essential part of the equipment of either the aerial photographer or
the designer of aerial photographic apparatus is a working knowledge of
the principles and construction of the airplane, and considerable actual
experience in the air. Conditions and requirements in the flying plane
are far different from those of the shop bench or photographic studio.
As a preliminary to undertaking any work on airplane instruments a good
text-book on the principles of flight should be studied. Such general
ideas as are necessary for understanding the purely photographic
problems are, however, outlined in the next paragraphs.

[Illustration:

  FIG. 1.—The elements of the plane.]
=Construction of the Airplane.=—The modern airplane (Fig. 1) consists of
one or more _planes_, much longer across than in the direction of flight
(_aspect ratio_). These are inclined slightly upward toward the
direction of travel, and their rapid motion through the air, due to the
pull of the _propeller_ driven by the _motor_, causes them to rise from
the earth, carrying the _fuselage_ or body of the airplane. In the
fuselage are carried the pilot, observer, and any other load. Wheels
below the fuselage forming the _under-carriage_ or _landing gear_ serve
to support the body when running along the ground in taking off or
landing. The pilot, sitting in one of the _cockpits_, has in front of
him the _controls_, by means of which the motion of the plane is guided
(Figs. 2 and 3). These consist of the engine controls—the _contacts_ for
the ignition, the _throttle_, the oil and gasoline supply, air pressure,
etc., and the steering controls—the _rudder bar_, the _stick_ and the
_stabilizer control_. The rudder bar, operated by the feet, controls
both the _rudder_ of the plane, which turns the plane to right or left
in the air, and the _tail skid_, for steering on the ground. The _stick_
is a vertical column in front of the pilot which, when pushed forward or
back, depresses or raises the _elevator_ and makes the machine dive or
climb. Thrown to either side it operates the _ailerons_ or wing tips,
which cause the plane to roll about its fore and aft axis. The
stabilizer control is usually a wheel at the side of the cockpit, whose
turning varies the angle of incidence of the small stabilizing plane in
front of the elevator, to correct the balance of the plane under
different conditions of loading.

[Illustration:

  FIG. 2.—Forward cockpit of DeHaviland 4, showing instrument board.]

[Illustration:

  FIG. 3.—Rear cockpit of DeHaviland 4, showing rear “stick” and rudder
    bar.]

The _fuselage_ consists usually of a light hollow framework of spruce or
ash, divided into a series of bays or compartments by upright members,
connecting the _longerons_, which are the four corner members, running
fore and aft, of the plane. Diagonally across the sides and faces of
these bays are stretched taut piano wires, and the whole structure is
covered with canvas or linen fabric. Cross-wires and fabric are omitted
from the top of one or more bays to permit their being used as cockpits
for pilot and observer. In later designs of planes the wire and fabric
construction has been superseded by ply-wood veneer, thereby
strengthening the fuselage so that many of the diagonal bracing wires on
the inside are dispensed with. This greatly increases the accessibility
of the spaces in which cameras and other apparatus must be carried.

[Illustration:

  FIG. 4.—Biplane in flight.]

The fuselage differs greatly in cross-section shape and in roominess
according to the type of engine. In the majority of English and American
planes, with their vertical cylinder or V type engines, the fuselage is
narrow and rectangular in cross-section. In many French planes, radial
or rotary engines are used and the fuselage is correspondingly almost
circular, and so is much more spacious than the English and American
planes of similar power. The shape and size of the plane body has an
important bearing on the question of camera installation.

[Illustration:

  FIG. 5.—A single-seater.]

=Types of Planes.=—The most common type of plane is the _biplane_ (Fig.
4), with its two planes, connected by struts and wires, set not directly
over each other, but _staggered_, usually with the upper plane leading.
_Monoplanes_ were in favor in the early days of aviation, and
_triplanes_ have been used to some extent. According to the position of
the propeller planes are classified as _tractors_ or _pushers_, tractors
being at present the more common form. Planes are further classified as
_single-seaters_ (Fig. 5), _two-seaters_, and _three-seaters_. These
motor and passenger methods of classification are now proving inadequate
with the rapid development of planes carrying two, three, and even more
motors, divided between pusher and tractor operation, and carrying
increasingly large numbers of passengers. Aside from structure, planes
may be further classified according to their uses, as _scout_, _combat_,
_reconnaissance_, _bombing_, etc. Planes equipped with floats or
pontoons for alighting on the water are called _seaplanes_ (Fig. 182),
and those in which the fuselage is boat-shaped, to permit of floating
directly on the water, are _flying boats_ (Fig. 183).

=The Plane in the Air.=—The first flight of the photographic observer or
of the instrument expert who is to work upon airplane instruments is
very profitably made as a “joy ride,” to familiarize him with conditions
in the air. His experience will be somewhat as follows:

The plane is brought out of the hangar, carefully gone over by the
mechanics, and the engine “warmed up.” The pilot minutely inspects all
parts of the “ship,” then climbs up into the front cockpit. He wears
helmet and goggles, and if the weather is cold or if he expects to fly
high, a heavy wool-lined coat or suit, with thick gloves and moccasins,
or an electrically heated suit. The passenger, likewise attired, climbs
into the rear cockpit and straps himself into the seat. He finds himself
sitting rather low down, with the sides of the cockpit nearly on a level
with his eyes. To either side of his knees and feet are taut wires
leading from the controls to the elevator, stabilizer, tail skid and
rudder. If the machine is dual control, the stick is between his knees,
the rudder bar before his feet. None of these must he let his body
touch, so in the ordinary two-seater his quarters are badly cramped.

At the word “contact” the mechanics swing the propeller, and, sometimes
only after several trials, the motor starts, with a roar and a rush of
wind in the passenger's face. After a moment's slow running it is
speeded up, the intermittent roar becomes a continuous note, the plane
shakes and strains, while the mechanics hold down the tail to prevent a
premature take-off. When the engine is properly warmed up it is
throttled to a low speed, the chocks under the wheels are removed, the
mechanics hold one end of the lower wing so that the plane swings around
toward the field. It then “taxis” out to a favorable position facing
into the wind with a clear stretch of field before it. After a careful
look around to see that no other planes are landing, taking off, or in
the air near by, the pilot opens out the engine, the roar increases its
pitch, the plane moves slowly along the ground, gathers speed and rises
smoothly into the air. Near the ground the air is apt to be “bumpy,” the
plane may drop or rise abruptly, or tilt to either side. The pilot
instantly corrects these deviations, and the plane continues to climb
until steadier air is reached.

At first the passenger's chief impressions are apt to be the deafening
noise of the motor, the heavy vibration, the terrific wind in his face.
If he raises his hand above the edge of the cockpit he realizes the
magnitude of wind resistance at the speed of the plane, and hence the
importance of the _stream-line section_ of all struts and projecting
parts.

When he reaches the desired altitude the pilot levels off the plane and
ceases to climb. Now his task is to maintain the plane on an even keel
by means of the controls, correcting as soon as he notes it, any
tendency to “pitch,” to “roll” or to “yaw” off the course. The resultant
path is one which approximates to level straight flying to a degree
conditioned by the steadiness of the air and the skill of the pilot. If
he is not skilful or quick in his reactions he may keep the plane on its
level course only by alternately climbing and gliding, by flying with
first one wing down and then the other, by pointing to the right and
then to the left. The skilled photographic pilot will hold a plane level
in both directions to within a few degrees, but he will do this easily
only if the plane is properly balanced. If the load on the plane is such
as to move the center of gravity too far forward with respect to the
_center of lift_ the plane will be _nose-heavy_, if the load is too far
back it will be _tail-heavy_. Either of these conditions can be
corrected, at some cost in efficiency, by changing the inclination of
the stabilizer. When the plane reaches high altitudes in rare air, where
it can go no further, it is said to have reached its _ceiling_. It here
travels level only by pointing its wings upward in the climbing
position, so that the fuselage is no longer parallel to the direction of
flight. An understanding of these peculiarities of the plane in flight
is of prime importance in photographic map making, where the camera
should be accurately vertical at all times.

The direction and velocity of the wind must be carefully considered by
the pilot in making any predetermined course or objective. The progress
of the plane due to the pull of the propeller is primarily with
reference to the air. If this is in motion the plane's _ground speed_
and direction will be altered accordingly. In flying with or against the
wind the ground speed is the sum or difference, respectively, of the
plane's _air speed_ (determined by an air speed indicator) and the speed
of the wind. If the predetermined course lies more or less across the
wind the plane must be pointed into the wind, in which case its travel,
with respect to the earth, is not in the line of its fore and aft axis.
The effect of “crabbing,” as it is called, on photographic calculations
is discussed later (Figs. 136 and 138).

When the plane has reached the end of its straight course and starts to
turn, its level position is for the moment entirely given up in the
operation of _banking_ (Fig. 6). Just as the tracks on the curve of a
railroad are raised on the outer side to oppose the tendency of the
train to slip outward, so the plane must be tilted, by means of the
ailerons, toward the inside of the turn. A point to be clearly kept in
mind about a bank is that if correctly made a plumb line inside the
fuselage will continue to hang vertical _with respect to the floor of
the plane, and not with respect to the earth_, for the force acting on
it is the combination of gravity and the acceleration outward due to the
turn. Only some form of gyroscopically controlled pointer, keeping its
direction in space, will indicate the inclination of the plane with
respect to the true vertical. If the banking is insufficient the plane
will _side slip_ outward or _skid_; if too great, it will side slip
inward.

[Illustration:

  FIG. 6.—Banking.]

As part of the “joy ride” the pilot may do a few “stunts,” such as a
“stall,” a “loop,” a “tail spin,” or an “Immelman.” From the
photographic standpoint these are of interest in so far as they bear on
the question of holding the camera in place in the plane. The thing to
be noted here is that (particularly in the loop), if these maneuvers are
properly performed, there is little tendency toward relative motion
between plane and apparatus. In a perfect loop it would, for instance,
be unnecessary, due to the centrifugal force outward, for the observer
to strap himself in. It is, however, =un=wise to place implicit
confidence in the perfection of the pilot's aerial gymnastics. No
apparatus should be left entirely free, although, for the reason given,
comparatively light fastenings are usually sufficient.

When nearing the landing field the pilot will throttle down the engine
and commence to glide. If he is at a considerable altitude he may come
down a large part of the distance in a rapid spiral. As the earth is
approached the air pressure increases rapidly, and the passenger, if
correctly instructed, will open his mouth and swallow frequently to
equalize the air pressure on his ear drums. Just before the ground is
reached the plane is leveled off, it loses speed, and, if the landing is
perfect, touches and runs along the ground without bouncing or bumping.
Frequently, however, the impact of the tail is sufficiently hard to
cause it to bump badly, with a consequent considerable danger to
apparatus of any weight or delicacy. This is especially apt to occur in
hastily chosen and poorly leveled fields such as must often be utilized
in war or in cross-country flying.

=Appearance of the Earth from the Plane.=—The view from the ordinary
two-seater is greatly restricted by the engine in front and by the
planes to either side and below (Figs. 7, 8, and 9). By craning his neck
over the side, or by looking down through an opening in the floor, the
passenger has an opportunity to learn the general appearance of the
subject he is later to devote his attention to photographing. Perhaps
the most striking impression he receives will be that of the _flatness_
of the earth, both in the sense of absence of relief and in the sense of
absence of extremes of light and shade. The absence of relief is due to
the fact that at ordinary flying heights the elevations of natural
objects are too small for the natural separation of the eyes to give any
stereoscopic effect. The absence of extremes of light and shade is in
part due to the fact that the natural surfaces of earth, grass and
forest present no great range of brightness; in part to the small
relative areas of the parts in shadow; in considerable part to the layer
of atmospheric haze which lies as an illuminated veil between the
observer and the earth at altitudes of 2000 meters and over (Figs. 10
and 11). Due to the combination of these factors the earth below
presents the appearance of a delicate pastel.

As the gaze is directed away from the territory directly below, the
thickness of atmosphere to be pierced rapidly increases, until toward
the horizon (which lies level with the observer here as on the ground)
all detail is apt to be obliterated to such an extent that only on very
clear days can the horizon itself be definitely found or be
distinguished from low lying haze or clouds (Fig. 4).

[Illustration:

  FIG. 7.—The view ahead.]

[Illustration:

  FIG. 8.—The view astern.]

=Airplane Instruments.=—Mounted on boards in front of the pilot and
observer are various instruments to indicate the performance of engine
and plane (Fig. 2). Those of interest to the photographic observer are
the _compass_, the _altimeter_, the _air speed indicator_, the
_inclinometers_.

[Illustration:

  FIG. 9.—The view between the wings.]

[Illustration:

  FIG. 10.—Appearance of the earth from a low altitude—3000 feet or
    less.]

The _compass_ is usually a special airplane compass, with its “card”
immersed in a damping liquid. Like most of the direction indicating
instruments on a plane its indications are only of significance when the
plane is pursuing a steady course. On turns or rapid changes of
direction of any sort perturbations prevent accurate reading.

The _altimeter_ is of the common aneroid barometer type. On American
instruments it is usually graduated to read in 100-foot steps. While
somewhat sluggish, it is quite satisfactory for all ordinary
determinations of altitude in photographic work. Were primary map making
to be undertaken, where the scale was determinable only from the
altitude and focal length of the lens, the ordinary altimeter is hardly
accurate enough.

[Illustration:

  FIG. 11.—Appearance of the earth from a high altitude—10,000 feet or
    more.]

The _air speed indicator_ consists of a combination of Venturi and Pitot
tubes, producing a difference of pressure when in motion through the air
which is measured on a scale calibrated in air speed. This instrument is
important for determining, in combination with wind speed, the _ground
speed_ of the plane, on the basis of which is calculated the interval
between exposures to secure overlapping photographs. Its accuracy is
well above that necessary for the purpose.

_Inclinometers_ for showing the lateral and fore and aft angle of the
plane with the horizontal, are occasionally used, and have also been
incorporated in cameras. The important point to remember about these
instruments is that they are controlled not alone by gravity but as well
by the acceleration of the plane in any direction. They consequently
indicate correctly only when the plane is flying straight. On a bank the
lateral indicator continues to indicate “vertical” if the bank is
properly calculated for the turn.




                                   II
                          THE AIRPLANE CAMERA




                              CHAPTER III
                   THE CAMERA—GENERAL CONSIDERATIONS


=Chief Uses of an Airplane Camera.=—The kinds of camera suitable for
airplane use and the manner in which they must differ from cameras for
use on the ground are determined by consideration of the nature of the
work they must do. Four kinds of pictures constitute the ordinary
demands upon the aerial photographer. These are single objectives or
_pin points_, _mosaic maps_ of strips of territory or large areas,
_oblique views_, and _stereoscopic views_. Each of these presents its
own peculiar problems influencing camera design.

_Pinpoints_ consist of such objects as gun emplacements, railway
stations, ammunition dumps, and other objects of which photographs of
considerable magnification are desired for study. Here the instrumental
requirements are _sufficient focal length of lens_ to secure an image of
adequate size; _means for pointing_ the camera accurately; enough
_shutter speed_ to counterbalance the speed of the plane; sufficiently
_wide lens aperture_ to give adequate exposure with the shutter speed
required; _means of supporting_ the camera to protect it from the
vibration of the plane.

_Mosaic maps_ are built up from a large number of photographs of
adjacent areas. In addition to the above requirements, mosaic maps
demand _lenses free from distortion_ and covering as large a plate as
possible, in order to keep to a minimum the number of pictures needed to
cover a given area; means for keeping the camera _accurately vertical_,
and means for _changing the plates or films_ and resetting the shutter
rapidly enough to avoid gaps between successive pictures. At low
altitudes and high ground speeds the interval between exposures becomes
a matter of only a few seconds.

_Oblique views_ are made at angles of from 12 to 35 degrees from the
horizontal, usually from comparatively low altitudes. They have been
found to be particularly suitable for the use of men who have no
training in photographic interpretation, being more like the pictures
with which the men are familiar. Distributed among the infantry before
an attack, they have proved indispensable aids to the proper knowledge
of the ground to be covered. The additional requirement here is for
_high shutter speed_ to eliminate the effect of the relatively very
rapid movement of the foreground.

_Stereoscopic views_ are among the most useful of all airplane pictures.
They are made from successive exposures, the separation of the points of
view being obtained not by two lenses at the distance of the eyes apart,
but by the motion of the plane. For this purpose the views should
overlap by at least 60 per cent; this, therefore, requires a very _short
interval between exposures_. For stereo-oblique views this may mean that
they are taken at intervals as short as one or two seconds.

=Chief Differences between Ground and Air Cameras.=—Certain definite
differences are thus seen to stand out between airplane cameras and the
ordinary kind. It is essential that the apparatus for use in the air
shall have high lens and shutter speed, means for rapid changing of
plates, and anti-vibration suspension. Without these features a camera
is of little use for aerial work. These requirements lead inevitably to
greater complexity of design. One simplification over ground cameras,
however, is brought about by the fact that all exposures are made on
objects beyond the practical infinity point of the lens; consequently,
all cameras are fixed focus. This fixed focus feature is a positive
advantage in construction, since it permits of the simple rigid box
form, desirable and necessary to withstand the strains due to the weight
of the lens and the stresses from the plane. But with the abandonment of
all provision for focussing in the air must go special care that the
material used in constructing the camera body is as little subject as
possible to expansion and contraction with temperature, since there is
often a drop of 30 to 40 degrees Centigrade from ground to upper air.
The effect of change of temperature on focus will be treated in the
discussion of lenses.

In addition to these differences, we must keep in mind certain
requirements which are conditioned by the nature and place of aerial
navigation. Thus all mechanical devices which will fail to function at
the low temperatures and pressures met at high altitudes are entirely
unsuitable. Experience has shown, too, that we must avoid all mechanism
depending primarily on _springs_ and on the action of _gravity_.
Vibration, and the motion of the plane in all three dimensions, conspire
to render mechanical motions unreliable when actuated by these agencies.
All plate changing, shutter setting, and exposing operations should be
as nearly as possible positively controlled motions. Because of the cold
of the upper air all knobs, levers and catches must be made extra large
and easy to handle with heavy gloves. Circular knurled heads to such
parts as shutter setting movements are to be avoided in favor of
bat-wing keys or levers. Grooves for the reception of magazines must be
as large and smooth as possible, and guides to facilitate the magazines'
introduction should be provided (Fig. 50). No releases or adjustments
which depend upon hearing or upon a delicate sense of touch are feasible
in airplane apparatus. Wherever possible, large _visible_ indicators of
the stage of the cycle of operations should be provided. Loose parts are
to be shunned, as they are invariably lost in service. Complete
operating instructions should be placed on the apparatus wherever
possible, to minimize the confusion due to changing and uninstructed
personnel.

=The Elements of the Airplane Camera.=—Disregarding its means of
suspension, the airplane camera proper consists essentially of _lens_,
_camera body_, _shutter_, and _plate or film holding and changing box_.

In certain of the aerial cameras developed early in the war all of these
elements were built together in a common enclosure. Later it was
generally recognized that a unit system of interchangeable parts is
preferable. In the case of the lens there arose various requirements for
focal length, from 25 to 120 centimeters, according to the work to be
done. Rather than use an entirely different camera for each different
kind of work, it is better to have lenses of various focal lengths,
mounted in tubes or cones, all built to attach to the same camera body.
In the case of the shutter it is desirable to be able to repair or
calibrate periodically. By making the shutter a removable unit, the
provision of a few spares does away with the need for putting the whole
camera out of commission. Similar considerations hold with reference to
other parts.

A further material advantage that comes from making airplane cameras in
sections is the greater ease with which they are inserted in the plane,
usually through the openings between diagonal cross-wires. It is in fact
only by virtue of this possibility of breaking up into small elements
that some of the larger cameras could be inserted in the common types of
reconnaissance plane. Illustrations of the building up of cameras from
separate removable elements are given in the detailed discussion of the
individual types.

=Types of Airplane Cameras.=—During the course of the war airplane
cameras have been classified on various bases, in different services. In
the French service, where the deMaria type of camera was standardized
early in the war, the usual classification was based on focal length;
thus the standard cameras were spoken of as the 26, the 50 and the 120
(centimeter). A further distinction was then made according to the size
of plate, this being originally 13 × 18 centimeters for the 26
centimeter, and 18 × 24 centimeters for the larger cameras. In the
English service the 4 × 5 inch plate was used almost exclusively, and
their various types of cameras were known by serial letters—C, E, L,
etc. Both these modes of classification became inadequate with the
ultimate agreement to standardize on the 18 × 24 centimeter size for all
plates, and to carry lenses of all focal lengths in interchangeable
elements.

For purposes of description and discussion, it is most convenient to
classify cameras according to their method of operation and the
sensitive material employed. On this basis we may distinguish among
cameras using _plates_ three kinds—_non-automatic cameras_,
_semi-automatic cameras_, and _automatic cameras_. We may similarly
discuss _film cameras_, but having treated the plate cameras
comprehensively, it will be found that the discussion of all types of
film camera can be handled most conveniently by studying the differences
in construction and operation introduced by the characteristics of film
as compared to plates.




                               CHAPTER IV
                     LENSES FOR AERIAL PHOTOGRAPHY


=General Considerations.=—The design and selection of lenses for aerial
photography present on the whole no problems not already encountered in
photography of the more familiar sort. Indeed, the lens problem in the
airplane camera is in some particulars more simple than in the ground
camera. For instance, there is no demand for depth of focus—all objects
photographed are well beyond the usually assumed “infinity focus” of
2000 times the lens diameter. Such strictly scientific problems of
design as pertain to aerial photographic lenses are ones of degree
rather than of kind. Larger aperture, greater covering power, smaller
distortion, more exquisite definition—these always will be in demand,
and each progressive improvement will be reflected in advances in the
art of aerial photography. But many lens designs perfected before the
war were admirably suited, without any change at all, for aerial
cameras.

Of the utmost seriousness, however, with the Allies, was the problem of
securing lenses of the desired types in sufficient numbers. The
manufacture of the many varieties of optical glass essential to modern
photographic lenses was almost exclusively a German industry, which had
to be learned and inaugurated in Allied countries since 1914. In
consequence of this entirely practical problem of quantity production
without the glasses for which lens formulæ were at hand, some new lens
designs were produced. Whether any of these possess merits which will
lead them to be preferred over pre-war designs, when the latter can
again be manufactured, remains to be seen.

While the glass problem was still unsolved, aerial cameras had to be
equipped with whatever lenses could be secured by requisition from
pre-war importation and manufacture, and later, with lenses designed to
utilize those glasses whose manufacture had been mastered in the allied
countries. It is important that the historical aspect of this matter be
well understood by the student of aerial photographic methods, for the
use of these odd-lot lenses reacted on the whole design of aerial
cameras and on the methods of aerial photography, particularly in
England and the United States. Almost without exception the available
lenses were of short focus, considered from the aerial photographic
standpoint; that is, they lay between eight and twelve inches. This set
a limit to the size of the airplane camera, quite irrespective of the
demands made by the nature of the photographic problem. Lenses of these
focal lengths produced images which, for the usual heights of flying,
were generally considered too small, and which were, therefore, almost
always subsequently enlarged. Such was the English practice, which was
followed in the training of aerial photographers in America, where
exactly similar conditions held at the start with respect to available
lenses. French glass and lens manufacturers did succeed in supplying
lenses of longer focus (50 centimeters), in numbers sufficient for their
own service, although never with any certainty for their allies. The
French, therefore, almost from the start, built their cameras with
lenses of long focus, and made contact prints from their negatives.

Practices adopted under pressure of an emergency to meet temporary
practical limitations often come to dominate the whole situation. This
is particularly true of aerial photography in the British and American
services. The small apparatus built around the stop-gap short focus
lenses fixed the plane designer's idea of an airplane camera, and the
space it should occupy. This was directly reflected in the designs of
the English planes, and the American planes copied after them. Meanwhile
the American photographic service in France associated itself with the
French service, adopting its methods and apparatus, and using French
planes whose designs were not being followed in American construction.
The task of harmonizing the photographic practice as taught in America,
following English lines, with French practice as followed in the theater
of war, and of adapting planes built on English designs so that they
could carry French apparatus, was a formidable one, not likely to be
soon forgotten by any who had a part in it.

=Photographic Lens Characteristics.=—Whole volumes have been written on
the photographic lens, and on the optical science utilized and indeed
brought into being by its problems. Such works should be consulted by
those who intend to make a serious study of the design of lenses for
aerial use. No more can be attempted, no more indeed is relevant here,
than an outline review of the chief characteristics and errors of
photographic lenses, considering them with special reference to aerial
needs.

The modern photographic lens is, broadly speaking, a development of the
simple convex or converging lens. Its function is the same: to form a
real image of objects placed before it. But the difference in
performance between the simple lens and the modern photographic
objective is enormous. The simple lens forms a clear image only close to
its axis, for light of a single color, and as long as its aperture is
kept quite small as compared to the distance at which the image is
formed. The photographic lens, on the other hand, is called upon to
produce a clear image with light of a wide range of spectral
composition, sharply defined over a flat surface of large area, and it
must do this with an aperture that is large in comparison with the focal
length, whereby the amount of light falling on the image surface shall
be a maximum. This ideal is approximated to a really extraordinary
degree by the scientific combination and arrangement of lens elements
made from special kinds of glass in the best photographic lenses of the
anastigmat type. The result is of necessity a set of compromises,
whereby the outstanding errors are reduced to a size judged permissible
in view of the work the lens is to do. These errors or _aberrations_ are
briefly reviewed below, in order that the reader may readily grasp the
terms in which the performance and tolerances in aerial lenses are
described.

[Illustration:

  FIG. 12.—Diagrammatic representation of spherical aberration.]

_Spherical Aberration and Coma._—Suppose we focus on a screen, by means
of a simple convex lens the image of a distant point of light. Suppose
for simplicity that this image is located on the axis of the lens and
that light of only one color is used, such as yellow. It will be found
that the smallest image that can be obtained is not a point, but a small
disc. This is due to the fact that the rays of light passing through the
outer portions of the lens are bent more than those passing through the
lens in the region near the center. This effect is shown in Fig. 12 by
the usual mode of representing it graphically. Here the figures 1, 2, 3,
4, represent distances from the axis of the lens, and the letters A_{1},
A_{2}, A_{3}, A_{4}, the points of convergence of the rays from 1, 2, 3,
4, etc. These distances projected upward on to the produced lens points
form a curve which shows at a glance the extent and direction of the
error due to each part of the lens. This information is of value where
the lens is fitted with an adjustable diafram. With some types of
correction sharper definition may be obtained by reducing the aperture.
With others, however, diaframing impairs definition, by destroying the
balance between under and over correction which averages to make a good
image. In aerial lenses it is not customary to use diaframs, as all the
light possible is desired. Consequently the reduction of spherical
aberration must be accomplished by proper choice of lens elements and
their arrangement.

Off the axis of the lens the image of a point source takes on an
irregular shape, due to oblique spherical aberration or _coma_.

_Chromatic Aberration._—Because of the inherent properties of the glass
of which it is made, a simple collective lens does not behave in the
same way with respect to light of different colors. If one attempts,
with such a lens, to focus upon a screen the image of a distant white
light, it will be found that the blue rays will not focus at the same
point as the red rays, but will come together nearer the lens. Modern
photographic lenses are compounded of two or more kinds of glass in such
a way as to largely eliminate this defect, the presence of which is
detrimental to good definition. Such lenses are called achromatic, and
the property of a lens by virtue of which this defect is eliminated is
called its _chromatic correction_.

Chromatic correction is never perfect, but two colors of the spectrum
can be brought to a focus in the same plane, and to a certain extent the
departure of other colors from this plane can be controlled. Off the
axis of the lens outstanding chromatic aberration results in a
difference in the size of images of different colors, known as _lateral
chromatism_.

Like spherical aberration, chromatic aberration is a contributing factor
to the size of the image of a point source, which determines the
defining power of a lens. It is, however, an error whose effect is to
some extent dependent on the kind of sensitive plate used. Two lenses
may give images of the same size (in so far as it is governed by
chromatic aberration), if a plate of narrow spectral sensitiveness is
used, while giving images of different size on panchromatic plates of
more extended color sensibility. The choice of the region of the
spectrum for which chromatic correction is to be made is thus governed
by the color of the photographically effective light. While in ordinary
photography the blue of the spectrum is most important, in aerial work
where color filters are habitually used with isochromatic plates the
green is most important, and color correction centered about this region
constitutes a real difference of design peculiar to aerial lenses.
Similarly the general use of deep orange or red filters with red
sensitive plates, for heavy mist penetration, would call for a shift of
correction to that part of the spectrum.

_Astigmatism and Covering Power._—Suppose the lens forms at some point
off its axis an image of a cross. Suppose one of the elements of the
cross to be on a radius from the center of the field, the other element
parallel to a tangent. The rays forming the images of these two elements
of the cross are subject to somewhat different treatment in their
passage through the lens. The curvature of the lens surfaces is on the
whole greater with respect to the rays from the radial element than to
those from the tangential element. They are therefore refracted more
strongly and come to a focus nearer the lens. The arms of the cross are
consequently not all in focus at once. This error, termed _astigmatism_,
is rather well shown in Fig. 15, where the images of the outlying
concentric circles are sharp in the radial, but blurred in the
tangential direction.

Astigmatism can be largely compensated for, and its character
controlled. The most usual correction brings the two images in focus
together both at the axis, and on a circle at some distance out. This
second locus of coincidence may or may not be in the same plane as the
first, depending on which disposition produces the best average
correction. The mean between the two foci determines the focal plane of
the lens, which is in general somewhat curved. The _covering power_ of a
lens is given by the size of the field which is sufficiently flat and
free from astigmatism for the purpose for which the lens is used. This
is largely determined by the astigmatism, but the other aberrations are
also important.

_Illumination._—The amount of light concentrated by the lens on each
elementary area of the image determines its brightness or illumination.
The ideal image would, of course, be equally bright over its whole area
of good definition, and for lenses of narrow angle this is approximately
true. But when it is desired to cover a wide angle the question of
illumination becomes serious. The relationship between angle from the
axis and illumination is that illumination is proportional to the fourth
power of the cosine of the angle. This relationship is shown in the
following table:

                         Angle Image brightness
                            0°  100   per cent.
                           10°   94.1 per cent.
                           20°   78.0 per cent.
                           30°   56.2 per cent.
                           40°   34.4 per cent.
                           50°   17.1 per cent.

If the field of view is 60°, which corresponds to an 18 × 24 centimeter
plate with a lens of 25 centimeter focus, the brightness is only 56 per
cent., and the necessary exposure at the edge approximately 1.8 times
that at the center. This effect is shown in Fig. 15. It is very
noticeable if the exposure is so short as to place the outlying areas in
the under-exposure period.

[Illustration:

  FIG. 13.—Barrel and pin-cushion distortion.]

_Distortion._—Sometimes a lens is relatively free from all the
aberrations, mentioned above, so that it gives sharp, clear images on
the plate, yet these images may not be exactly similar to the objects
themselves as regards their geometrical proportions; in other words, the
image will show distortion. Lens distortion assumes two typical forms,
illustrated in Fig. 13, which shows the result of photographing a square
net-work with lenses suffering in the one case from “barrel” distortion
and in the other from “pin-cushion” distortion. In the first the corners
are drawn in relative to the sides; in the latter case the sides are
drawn in with respect to the corners. Either sort is a serious matter in
precision photography, such as aerial photographic mapping aspires to
become. It must be reduced to a minimum and its amount must be
accurately known if negatives are to be measured for the precise
location of photographed objects. In general symmetrical lenses give
less distortion than the unsymmetrical (Fig. 14).

[Illustration:

  FIG. 14.—Arrangement of elements in two lenses suitable for aerial
    work: _a_, Zeiss Tessar; two simple and one cemented components
    (unsymmetrical); _b_, Hawkeye Aerial; two positive elements of heavy
    barium crown, two negative of barium flint, uncemented
    (symmetrical).]

=Lens Testing and Tolerances for Aerial Work.=—Simple and rapid
comparative tests of lenses may be made by photographing a _test chart_,
consisting of a large flat surface on which are drawn various
combinations of geometrical figures—lines, squares, circles,
etc.—calculated to show up any failures of defining power. For testing
aerial lenses the chart should be as large as possible, so that it may
be photographed at a distance great enough for the performance of the
lens to be truly representative of its behavior on an object at infinite
distance. This means in practice a chart of 4 or 5 meters side, to be
photographed at a distance 20 to 30 times the focal length of the lens.

[Illustration:

  FIG. 15.—Photograph of a lens testing chart, showing failure in
    defining power outside area for which the lens is calculated.]

A typical photograph of such a chart is shown in Fig. 15. It reveals at
a glance the more conspicuous lens errors. At the sides and corners the
concentric circles show the lens's astigmatism, by the clear definition
of the lines radial to the center of the field and their blurring in the
tangential direction. The falling off in illumination with increasing
distance from the center is also exhibited; and the blurring of all
detail outside the rectangle for which the lens was calculated shows
that spherical, chromatic, and other aberrations have become
prohibitively large.

But the only complete test of a lens is the quantitative measurement of
errors made on an optical bench. A point source of light, which may at
will be made of any color of the spectrum, is used as the object and its
image formed by the lens in a position where it can be accurately
measured for location, size, and shape by a microscope. A chart giving
the results of such a test is shown in Fig. 16. In the upper left-hand
corner is shown the position of the focus for the different colors of
the spectrum. Below this is recorded the lateral chromatism at 21
degrees, in terms of the difference in focus for a red and a blue ray.
Below this again comes the distortion, or shift of the image from its
proper position, for various angles (plotted at the extreme right) from
the lens axis. To the right of this is the image size, at each angle,
and finally, to the right of the diagram, are plotted the distances of
the two astigmatic foci from the focal plane, together with the mean of
the two foci, which practically determines the shape of the field.

An important point to notice is that these data are uniformly plotted in
terms of a lens of 100 millimeters focal length irrespective of the
actual focal length of the lens measured. Thus this particular chart is
for a 50 centimeter lens but would be plotted on the same scale for a 25
or a 100 centimeter lens. Underlying this practice is the assumption
that all the characteristics of lenses of the same design and aperture
are directly proportional to their focal length. If this were so, then a
50 centimeter lens would give double the size of image that a 25
centimeter does, and so on. As a matter of fact, test shows that the
size of the image does not increase so rapidly as the focal length; so
that while the image size for a 25 centimeter lens would be, say, .05
millimeters per 100 millimeters focal length, it will be only .03 or .04
millimeters per 100 millimeters focal length for a 50 centimeter lens.
The actual size of a point image will therefore be greater, though not
proportionately greater.

[Illustration:

  FIG. 16.—Chart recording measurements of lens characteristics.]

The chart presents tests on a good quality lens, and so gives a good
idea of the permissible magnitude of the various errors. In many ways
the most important figure is that for image size, including as it does
the result of all the aberrations. In the example given, this varies
from .075 to .15 mm. actual size. For the same type of lens of 25
centimeters focus this range will be from .05 to .10 mm. Since these are
commonly used focal lengths, a good average figure for image size,
commonly used in aerial photographic calculations, is ⅒ mm. In regard to
astigmatic tolerances, the two astigmatic foci should not be separated
at any point by more than 6 to 7 millimeters, and the mean of these
should not deviate from the true flat field by more than ½ millimeter,
in each case the figures being based on the conventional 100 millimeters
focal length. Distortion should not be over .08 millimeter at 18° or .20
millimeter at 24° from the axis (per 100 millimeters focal length).

=Lens Aperture.=—In the simple lens the aperture is merely the diameter.
In compound lenses the aperture is not the linear opening but the
effective opening of an internal diafram. Photographically, however,
aperture has come to have a more extensive meaning. While in the
telescope the actual diameter of an objective is perhaps the most
important figure, and in the microscope the focal length, in photography
the really important feature is the amount of light or illumination.
This is determined by lens opening and focal length together;
specifically, by the ratio of the lens area to the focal length. The
common system of representing photographic lens aperture is by the ratio
of focal length to lens diameter, the lens being assumed to be circular.
Thus F/5 (often written F.5) indicates that the diameter is one-fifth
the focal length.

Two points are to be constantly borne in mind in connection with this
system of representation. First, all lenses of the same aperture (as so
represented) give the same illumination of the plate (except for
differences due to loss of light by absorption and reflection in the
lens system). This follows simply from the fact that the illumination of
the plate is directly proportional to the square of the lens diameter,
and inversely as the square of the focal length. Secondly, the
illumination of the plate is inversely as the square of the numerical
part of the expression for aperture. That is, lenses of aperture F/4.5
and F/6 give images of relative brightness (6/4.5)^2 = 1.78.

What lens aperture, and therefore what image brightness, is feasible, is
determined chiefly by the angular field that must be covered with any
given excellence of definition. The largest aperture ordinarily used for
work requiring good definition and flat field free from distortion is
F/4.5. Anastigmatic lenses of this aperture cover an angle of 16° to 18°
from the axis satisfactorily, which corresponds to an 18 × 24 centimeter
plate with a lens of 50 centimeters focus. Lenses with aperture as large
as F/3.5 were used to some extent in German hand cameras of 25
centimeters focal length, with plates of 9 × 12 centimeters. English and
American lenses of this latter focal length were commonly of aperture
F/4.5, designed to cover a 4 × 5 inch plate.

As a general rule the greater the focal length the smaller the
aperture—a relationship primarily due to the difficulty of securing
optical glass in large pieces. Thus while 50 centimeter lenses of
aperture F/4.5 are reasonably easy to manufacture, the practicable
aperture for quantity production is F/6, and for 120 centimeter lenses,
F/10. This means that a very great sacrifice of illumination must be
faced to secure these greater focal lengths. As is to be expected from
the state of the optical glass industry, the German lenses were of
generally larger aperture for the same focal lengths than were those of
the Allies. Besides the F/3.5 lenses already mentioned, their 50
centimeter lenses were commonly of aperture F/4.8, their 120 centimeter
lenses of aperture F/7, or of about double the illuminating power of the
French lenses of the same size.

Demands for large covering power also result in smaller aperture. The 26
centimeter lenses used on French hand cameras utilizing 13 × 18
centimeter plates were commonly of aperture F/6 or F/5.6. The lens of
largest covering power decided on for use in the American service was of
12 inch focus, to be used with an 18 × 24 centimeter plate (extreme
angle 26°); the largest satisfactory aperture for this lens is F/5.6.

Ordinarily the question of aperture is closely connected with that of
diaframs, whereby the lens aperture may be reduced at will. Diaframs
have been very little used in aerial photography. All the aperture that
can be obtained and more is needed to secure adequate photographic
action with the short exposures required under the conditions of rapid
motion and vibration peculiar to the airplane. Any excess of light, over
the minimum necessary to secure proper photographic action, is far
better offset by increase of shutter speed or by introduction of a color
filter. For this reason American aerial lenses were made without
diaframs. In the German cameras, however, adjustable diaframs are
provided (Fig. 43), controlled from the top of the camera by a rack and
pinion. In the camera most used in the Italian service an adjustable
diafram is provided, but this is occasioned by the employment of a
between-the-lens shutter of fixed speed, so that the only way exposure
can be regulated is by aperture variation, a method which has little to
recommend it.

=The Question of Focal Length.=—In aerial photography the lens is
invariably used at fixed, infinity, focus. Under these conditions the
simple relationship holds that the size of the image is directly
proportional to the focal length and inversely proportional to the
altitude. If any chosen scale is desired for the picture the choice of
focal length is determined by the height at which it is necessary to
fly. This at least would be the case were there no limitation to the
practicable focal length—which means camera size—and were one limited to
the original size of the picture as taken; that is, were the process of
enlargement not available. But the possibility of using the enlarging
process brings in other questions: Is the defining power of a short
focus lens as good in proportion to its focal length as that of a long
focus lens? If so a perfect enlargement from a negative made by a short
focus lens would be identical with a contact print from a negative made
with a lens of longer focus. Is defining power lost in the enlarging
process with its necessary employment of a lens which has its own errors
of definition and which must be accurately focussed?

Certain factors which enter into comparisons of this sort in other lines
of work, such as astronomical photography, play little part here. These
are, first, the optical resolving power of the lens, which is
conditioned by the phenomena of diffraction, and is directly as the
diameter; and, second, the size of the grain of the plate emulsion. The
first of these does not enter directly, because the size of a point
image on the axis of the lens, due merely to diffraction, is very much
less than that given by any photographic lens which has been calculated
to give definition over a large field, instead of the minute field of
the telescope. Yet it may contribute toward somewhat better definition
with a long focus lens because of the actually larger diameter of such
lenses. The second factor is not important, because, as will be seen
later, the resolving power of the plates suitable for aerial photography
is considerably greater than that of the lens. The emulsion grain is in
fact only a quarter or a fifth the size of the image as given by a 25
centimeter lens, and enlargements of more than two or three times are
rarely wanted.

A series of experiments was made for the U. S. Air Service to test out
these questions, using a number of representative lenses of all focal
lengths, both at their working apertures and at identical apertures for
all. With regard to lens defining power, as shown by the size of a point
image, the answer has already been reported in a previous section.
Lenses of long focus give a relatively smaller image than lenses of the
same design of short focus. In regard to the whole process of making a
small negative and enlarging it, the loss of definition is quite marked,
as compared to the pictures of the same scale made by contact printing
from negatives taken with longer focus lenses.

This answer is clear-cut only for lenses calculated to give the same
angular field. Thus a 10 inch lens covering a 4 × 5 inch plate has about
the same angle as a 50 centimeter lens for an 18 × 24 centimeter plate.
When, however, it comes to the longer foci, such as 120 centimeters, the
practical limitation to plate size (18 × 24 cm.) has been passed, and
the angular field is less than half that of the 50 centimeter lens. The
120 centimeter lens need only be designed for this small angle, with
consequent greater opportunities for reduction of spherical aberration.
It is therefore an open question whether a 50 centimeter lens designed
to cover a plate of linear dimensions 50/120 times that used with the
regular 50 centimeter lens could not be produced of such quality that it
would yield enlargements equal to contacts from a 120 centimeter lens.
If so, lenses of larger aperture could be used, and a considerable
saving in space requirements effected.

Focal lengths during the Great War were decided by the nature of the
military detail which was to be revealed and by the altitudes to which
flying was restricted in military operations. In the first three years
of the war the development of defences against aircraft forced planes to
mount steadily higher, so that the original three or four thousand feet
were pushed to 15,000, 18,000, and even higher. Lenses of long focus
were in demand, leading ultimately to the use of some of as much as 120
centimeters (Fig. 41). In the last months of the war the resumption of
open fighting made minute recording of trench details of less weight,
while the preponderance of allied air strength permitted lower flying.
In consequence, lenses of shorter focus and wider angle came to the
fore, suitable for quick reconnaissance of the main features of new
country. At the close of the war the following focal lengths were
standard in the U. S. Air Service, and may be considered as well-suited
for military needs. Peace may develop quite different requirements.

                Focal length    Aperture    Plate size
                10 inch      F/4.5          4 × 5 inch
                26 cm.       F/6            13 × 18 cm.
                12 inch      F/5.6          18 × 24 cm.
                20 inch      F/6.3 to F/4.5 18 × 24 cm.
                48 inch      F/10 to F/8    18 × 24 cm.

The question of the use of _telephoto lenses_ in place of lenses of long
focus is frequently raised. Lenses of this type combine a diverging
(concave) element with the normal converging system, whereby the effect
of a long focus is secured without an equivalent lens-to-plate distance.
This reduction in “back focus” may be from a quarter to a half. Were it
possible to obtain the same definition with telephoto lenses as with
lenses of the same equivalent focus, they would indeed be eminently
suitable for aerial work because of their economy of length. But
experience thus far has shown that the performance of telephoto lenses,
as to definition and freedom from distortion, is distinctly inferior, so
that it is best to hold to the long focus lens of the ordinary type.

=Lenses Suitable for Aerial Photography.=—Among the very large number of
modern anastigmat lenses many were found suitable for airplane cameras
and were used extensively in the war. A partial list follows: The Cooke
Aviar, The Carl Zeiss Tessar, the Goerz Dogmar, the Hawkeye Aerial, the
Bausch and Lomb Series Ic and IIb Tessars, the Aldis Triplet, the
Berthiot Olor.

=The Question of Plate Size and Shape.=—Plate size is determined by a
number of considerations, scientific and practical. If the type of lens
is fixed by requirements as to definition, then the dimensions of the
plate are limited by the covering power. From the standpoint of economy
of flights and of ease of recognizing the locality represented in a
negative, by its inclusion of known points, lenses of as wide angle as
possible should be used. If the focus is long, this means large plates,
which are bulky and heavy. If the finest rendering of detail is not
required a smaller scale may be employed, utilizing short focus lenses
and correspondingly smaller plates. Thus a six inch focus lens on a 4 ×
5 inch plate would be as good from the standpoint of angular field as a
12 inch on an 8 × 10 inch plate. This is apt to be the condition with
respect to most peace-time aerial photography, which may be expected to
free itself quickly from the huge plates and cameras of war origin.

For work in which great freedom from distortion of any sort is
imperative, small plates will be necessary, for two reasons. One is that
the characteristic lens distortions are largely confined to the outlying
portions of the field. The other is that a wide angle of view inevitably
means that all objects of any elevation at the edge of the picture are
shown partly in face as well as in plan, which prevents satisfactory
joining of successive views (Fig. 128). In making a mosaic map of a
city, if a wide angle lens is employed with large plates, the buildings
lying along the junctions of the prints can be matched up only for one
level. If this is the ground level, as it would be to keep the scale of
the map correct, the roofs will have to be sacrificed. In extreme cases
a house at the edge of a junction may even show merely as a front and
rear, with no roof, while in any case the abrupt change at these edges
from seeing one side of all objects to seeing the opposite side is not
pleasing.

The table in a preceding section gives the relation of plate size to
focal length found best on the whole for military needs. Deviations from
these proportions in both directions are met with. In the English
service the LB camera, which uses 4 × 5 inch plates, is equipped with
lenses of various focal lengths, up to 20 inches. The German practice,
as well as the Italian, was almost uniform use of 13 × 18 centimeter
plates for all focal lengths. Toward the end of the war, however, some
German cameras of 50 centimeter focal length were in use employing
plates 24 × 30 centimeters.

It will be recognized that these plate sizes are chosen from those in
common use before the war. A similar observation holds with even greater
force on the question of plate _shape_. Current plate shapes have been
chosen chiefly with reference to securing pleasing or artistic effects
with the common types of pictures taken on the ground. These shapes are
not necessarily the best for aerial photography. Indeed the whole
question of plate shape should be taken up from the beginning, with
direct reference to the problems of aerial photography and photographic
apparatus.

A few illustrations will make this clear, taking Fig. 17 as a basis. If
it is desired to do spotting (the photography of single objectives), the
best plate shape would be circular, for that shape utilizes the entire
covering area of the lens. If it is desired to make successive
overlapping pictures, either for mapping, or for the production of
stereoscopic pairs, a rectangular shape is indicated. If the process of
plate changing is difficult or slow, it is advisable, in order to give
maximum time for this operation, to have the long side of the rectangle
parallel to the line of flight (indicated by the arrow). If economy of
flights is a consideration, as in making a mosaic map of a large area,
it is advantageous to have as wide a plate as the covering power of the
lens will permit. Reference to Fig. 17 shows that this means a plate of
small dimensions in the direction of flight. If the changing of plates
or film is quick and easy, the maximum use of the lens's covering power
is made by such a rectangle whose long side approximates the dimensions
of the lens field diameter. This is in fact the choice made in the
German film mapping camera (Figs. 61 and 63), whose picture is 6 × 24
centimeters. An objection to this from the pictorial side, lies in the
many junction lines cutting up the mosaic. Another objection, if the
plane does not hold a steady course, is the failure to make overlaps on
a turn. (Fig. 62.) Here as everywhere the problem is to decide on the
most practical compromise between all requirements.

[Illustration:

  FIG. 17.—Possible choices of plate shape.]

=Focussing.=—The process of focussing aerial cameras was at first deemed
a mystery, though undeservedly so. A belief was long current that
“ground” focus and “air” focus differ. In other words, that a camera
focussed upon a distant object on the ground would not be in focus for
an object the same distance below the camera when in the plane. Belief
in this mysterious difference went so far that certain instruction books
describe in detail the process of focussing a camera by trial exposures
from the air.

Careful laboratory tests performed for the U. S. Air Service showed that
neither low temperature nor low pressure, such as would be met at high
altitudes, alter the focus of any ordinary lens by a significant amount,
and that the possible contraction of the camera body was of negligible
effect on the focus (not more than 1/200 per cent. per degree centigrade
with a metal camera). In complete harmony with these tests has been the
experience that if the ground focussing is done carefully, by accurate
means, then the air focus is correct. The whole matter thus becomes one
of precision focussing.

The best method, applicable if the air is steady, is to focus by
_parallax_. The ground glass focussing screen is marked in the center
with a pencilled cross. Over this is mounted, with Canada balsam, a thin
microscope cover-glass. The camera is directed on an object a mile or
more away, and the image formed by the lens is examined by a magnifying
glass through the virtual hole formed by the affixed cover-glass. With
the pencil line in focus the head is moved from side to side. If the
image and pencil mark coincide they will move together as the head is
moved. If the image moves away from the pencil mark and in the _same_
direction as the eye moves, the image is too near the lens. If the image
moves away in the _opposite_ direction to the motion of the eye, it is
too far from the lens. In either case the focus is to be corrected
accordingly.

In place of a distant object, which may waver with the motion of the
air, we may use an image placed at infinity by optical means. The
_collimator_, an instrument for doing this, consists of a test object
(lines, circles, etc.) placed accurately at the focus of a telescope
objective. The camera lens is placed against this and focussed by
parallax, as with a distant object. Collimators are employed in camera
factories, and should be part of the equipment of base laboratories
where repairing and overhauling of cameras is done.

=Lens Mounts.=—All that is required for the mounting of an aerial camera
lens is a rigid platform, with provision for enough motion of the lens
to adjust its focus accurately. As already explained, the lens works at
fixed, infinity, focus, and therefore needs no adjustment during use. It
must be held far more rigidly than would be possible by the bellows,
which is an almost invariable adjunct of focussing cameras. The use of
ordinary types of hand cameras on a plane is rarely successful just
because of the bellows, which is strained and rattled by the rush of
wind.

The lens mountings thus far used have been simple affairs. In the French
cameras the lens is merely screwed into a flange which in turn is
fastened by screws to a platform in the camera body. Adjustment for
focussing is not provided; instead, the flange is raised on thin metal
rings or washers, cut of such thickness by trial as to bring the lens to
focus, once and for all.

The U. S. Air Service method of mounting is to provide the lens barrel
with a long thread, which screws into a flange that in turn is mounted
on a platform in the camera cone, by means of thumb-screws. The lens is
focussed by screwing in and out, and then clamped by a screw through the
side, bearing on the thread. The whole mount may be quickly removed by
loosening the thumb-screws, and once focussed in one cone, can be
transferred to another similar, machine-made cone without change of
focus. Fig. 18 shows a 20 inch lens mounted in this manner. The
photograph shows as well the ring on the front of the lens by means of
which circular color filters may be held in place. This ring screws down
on the filter, and the catch is dropped into the nearest vertical groove
to the tight position.

[Illustration:

  FIG. 18.—50 centimeter F/6 lens in U. S. standard mount, showing color
    filter retaining ring and catch.]

A somewhat different and better method of tightening the lens in the
flange, when focussed, has been adopted in the English lens mount, which
is in general similar to the American. The threaded part of the flange
is split by a slot cut parallel to the flange base, and a screw is run
into the flange from the front, through the split portion. By tightening
this screw, which is always accessible, the split part of the flange is
squeezed together, thus rigidly holding the lens barrel.




                               CHAPTER V
                              THE SHUTTER


=Permissible Exposure in Airplane Photography.=—A definite limitation to
the length of exposure in airplane cameras is set by the motion of the
plane. If we represent the speed of the plane by _S_, the altitude of
the plane by _A_, and the focal length of the lens by _F_, we obtain at
once from the diagram (Fig. 19), that _s_, the rate of movement of the
image on the plate, is given by the relation,

                               _s_   _F_
                               ——— = ———
                               _S_   _A_

If we call the permissible movement _d_, then the permissible exposure
time, _t_, is given by the relation—

                                  _d_   _Ad_
                            _t_ = ——— = ————
                                  _s_   _FS_

As a representative numerical case, expressing all quantities in
centimeters and in centimeters per second, let _F_ = 50, _S_ =
20,000,000/3600 (200 kilometers per hour), and _A_ = 300,000, then

                       50 × 20,000,000
                 _s_ = ——————————————— = .9 centimeters
                       300,000 × 3600

If we take for the permissible undetectable movement, .01 centimeter,
which is, as has been shown, a reasonable figure for lens defining
power, we have, then, that the _longest permissible exposure is .011
second_—in round numbers, one-hundredth.

In flying with a slow plane, or in flying against the wind, the exposure
can sometimes be increased to as much as double this length. Diminishing
_F_ would similarly extend the allowable exposure, but the ratio of _F_
to _A_ approximates to a constant in actual practice; in other words, a
certain resolution and size of image have been found desirable. If
flying is forced higher, a longer focus lens is used; if lower flying is
possible, a lens of shorter focus. This relationship has, of course,
been derived from war-time experience. Probably much of the prospective
peace-time mapping work will impose substantially easier requirements as
to definition and will thus allow longer exposures.

[Illustration:

  FIG. 19.—Relative motion of plane and photographic image.]

For low oblique views the longest exposure is much less. Taking 45
degrees as a representative angle for the foreground, and 500 meters as
a representative height, the value of _t_ becomes 1/600.

These figures will illustrate two important points: they show how severe
is the limitation as to exposure, with the consequent heavy demand on
lens and sensitive material speed; and they show how important it is to
secure a shutter with the maximum light-giving power for a specified
length of exposure. This leads to a study of the characteristics as to
efficiency of the two common types of shutter, namely, _shutters at or
between the lens_, and _focal-plane shutters_.

=Characteristics of Shutters Located at the Lens.=—Of the various
shutters located at the lens the most common is the type that is
clumsily but descriptively termed the “between-the-lens” shutter. This
is composed of thin hard rubber or metal leaves or sectors which overlap
and which are pulled open to make the exposure. It may require two
operations, one for setting and one for exposing, or it may, as in some
makes, set and expose by a single motion. Clock escapements, or some
form of frictional resistance, are depended on to control the interval
between opening and closing. This shutter is the one almost universally
employed on small hand cameras and on all lenses up to about two inches
diameter. It gives speeds sometimes marked as high as 1/300 second,
although usually not over 1/100 on actual test.

Between-the-lens shutters have been used to some extent on the shorter
focus (up to 25 centimeter) aerial cameras, notably in the Italian
service. They suffer, however, from two limitations. In the first place
we have not yet solved the mechanical problems met with in trying to
make the shutter of large size (as for 50 centimeter _F_/6 lenses) at
the same time to give high speeds. In the second place the efficiency of
the type is low because a large part of the exposure time is occupied by
the opening and closing of the sectors.

If we define the _efficiency_ of a shutter as the ratio of the amount of
light it transmits during the exposure to the amount of light it would
transmit were it wide open during the whole period, then the efficiency
of the ordinary between-the-lens shutter is of the order of 60 per cent.
This means 1.6 times the motion of the image for the same photographic
action that we should have with a perfect shutter. The accompanying
photographic record (Fig. 20) of the opening and closing process of this
type of shutter clearly illustrates its deficiencies.

[Illustration:

  FIG. 20.—Effective lens opening at equal intervals of time: (_a_)
    during focal plane shutter exposure; (_b_) during between-the-lens
    shutter exposure.]

=Characteristics of the Focal-Plane Shutter.=—Long before the days of
aerial photography the problem of a high-efficiency high-speed shutter
for photographing moving objects on the ground—railway trains or racing
automobiles—had already led to the development of the _focal-plane
shutter_. This is a type peculiarly adapted to the problems of the
airplane camera. It consists essentially of a curtain, running at high
speed close to the photographic plate, the exposure being given by a
narrow rectangular slot.

If the focal-plane shutter is in virtual contact with the sensitive
surface the efficiency, as defined above, is 100 per cent., since the
whole cone of rays from the lens illuminates the plate during the whole
time of exposure. But if the curtain is not carried close to the plate
the efficiency falls off rapidly with distance, especially so for small
apertures of the slot.

[Illustration:

  FIG. 21.—Calculation of focal plane shutter efficiency.]

_The efficiency of the focal-plane shutter_ may be calculated as
follows: Let the focal length of the lens be _F_, its diameter be _F/N_,
the width of the slot be _a_, and the distance from plate to curtain _d_
(Fig. 21). Now if the curtain is moving at a uniform speed, the time
taken for the slot to traverse the whole cone of rays, from the instant
it enters till the instant it leaves, will be directly proportional to

                      _d_ (_F_)         _d_
                      ——— (———) + _a_ = ——— + _a_
                      _F_ (_N_)         _N_

If the curtain were in contact with the plate the time taken for the
same amount of light to reach the sensitive surface would be
proportional to _a_. Again defining shutter efficiency as the ratio of
the light transmitted to what would have been transmitted were the
shutter fully open for the total time of exposure, the efficiency, _E_,
is given at once by the expression—

                                     _a_
                           _E_ = ———————————
                                 _d/N_ + _a_

As an example let the lens aperture be _F_/6, so that _N_ = 6; let _d_ =
1, and _a_ = 1, then _E_ = 6/7. In the French deMaria cameras, where _d_
= 4 centimeters, _E_ = 60 per cent. for the aperture assumed, which is
representative. Fig. 22 exhibits diagrammatically the chief
characteristics of the focal plane shutter.

[Illustration:

  FIG. 22.—Characteristics of focal plane shutter.]

In view of the necessity for _some_ distance between shutter and plate
it is obviously important to keep _a_ as large as possible, depending
for the requisite shutter speed on the velocity of the curtain. Large
aperture and high curtain speed are also found to be desirable when we
consider the distortion produced by the focal-plane shutter.

=Distortions Produced by the Focal-plane Shutter.=—While the time of
exposure of any point on the plate can, with the focal-plane shutter,
easily be made 1/100 second or less, the whole period during which the
shutter is moving is much greater than this. For instance, a 1
centimeter opening which gives 1/100 second exposure takes ⅒ second to
move across a 10 centimeter plate, or nearly ⅕ second for an 18
centimeter plate. With a moving airplane this means that the point of
view at the end of the exposure has moved forward compared to that at
the beginning, by the amount of motion of the plane in the interval. If
the shutter moves in the direction of motion of the plane the image will
be magnified; if in the opposite direction, it will be compressed along
the axis of motion. The amount of this distortion is calculated as
follows:

Let the velocity of the plane be _V_, and that of the shutter be _v_.
Let the focal length of the camera be _F_, and the altitude _A_. If the
camera were stationary, a plate of length _l_ would receive on its
surface an image corresponding to a distance _A/F_ × _l_ on the ground.
Due to the motion of the shutter the end of the exposure occurs at a
time _l/v_ after the start. In this time the plane has moved a distance
_V_ × _l/v_; hence the point photographed at the end of the shutter
travel is _Vl/v_ within or beyond the original space covered by the
plate, depending on the direction of motion of the curtain. The
distortion, _D_, is given by the ratio of this distance to the length
corresponding to the normal stationary field of view:

                              _V/v_ × _l_   _VF_
                        _D_ = ——————————— = ————
                              _A/F_ × _l_   _vA_

When _V_ = 200 kilometers per hour, _v_ = 100 centimeters per second,
_F_ = 50 centimeters, _A_ = 3000 meters, we have—

                     20,000,000 × 50                     1
             _D_ = ———————————————————— = approximately ———
                   3600 × 100 × 300,000                 100

Or if the actual distance error on the ground is desired,

                           _Vl_
                           ———— = 10.8 meters
                           _v_

As a percentage error this one per cent. is small compared with other
uncertainties, such as film shrinkage or the error of level of the
camera. As an absolute error in surveying, thirty feet is, of course,
excessive.

The distortion is diminished for any specified shutter speed by making
the speed of travel of the curtain as large as possible and by
correspondingly increasing the aperture. In connection with film
cameras, another solution which has been suggested is to move the film
continuously during the exposure in the direction of the plane's motion.
The requisite speed of the film _v'_ to eliminate distortion is given by
the relation:

                               _v'_   _F_
                               ———— = ———
                               _V_    _A_

For the values of _V_, _F_, and _A_ used above, _v'_ = .92 centimeters
per second. This speed is clearly that which holds the image stationary
on the film—a fact which suggests another object for such movement,
namely, to permit of longer exposures.

The effect of focal plane distortion may be averaged out in the making
of strip maps, if the shutter is constructed so as to move in opposite
directions on successive exposures. The first picture will be magnified,
the second compressed, and so on, but a strip formed of accurately
juxtaposed pictures will be substantially accurate in over-all length.
Such a shutter is embodied in one of the German film cameras (Fig. 61).

Distortion of the kind above discussed is absent with between-the-lens
shutters, which may conceivably be improved in efficiency and in
feasible size. If so they would merit serious consideration for aerial
mapping.

=Methods and Apparatus for Testing Shutter Performance.=—With a
focal-plane shutter the desirable qualities in performance are three in
number: (1) _Adequate speed range_, which may be taken as from 1/50 to
1/500 second for aerial work, (2) _good efficiency_, which has already
been treated, and (3) _uniformity of speed_ during its travel across the
plate. Before the advent of aerial photography little attention was paid
to speed uniformity, differences of 50 per cent. in initial and final
speed being common in focal-plane shutters, and but little noticed in
ordinary landscape work because of the natural variation of brightness
from sky to ground. In the making of aerial mosaic maps the
non-uniformity of density across the plate results in a most offensive
series of abrupt changes of tone at the junction points of the
successive prints (Fig. 140), an effect which must be minimized by
manipulation of the printing light.

Instruments for testing the speed and uniformity of action of
focal-plane shutters are an essential part of any laboratory for
developing or testing photographic apparatus and some simple device for
setting and checking shutter speed should be available in the field.
Every such speed tester must contain some form of time counting
element—pendulum, tuning fork or clock-work. Elaborate shutter testers,
suitable for determining all the characteristics of all types of
shutter, have been developed and used in certain of the photographic
research laboratories. For the study and setting of focal-plane shutters
(whose efficiency need not be measured, as it can be simply calculated
from linear dimensions), the following simple kinds of apparatus are
adequate:

[Illustration:

  FIG. 23.—Apparatus for testing focal plane shutter speed throughout
    the travel of the curtain.]

_Clock dial type of shutter tester._ This consists essentially of a
black clock dial carrying a white pointer which makes its complete
revolution in one second or less. If this dial is photographed by the
camera under test, the width of the sector traced during the exposure by
the moving pointer shows the time interval. If the dial is photographed
at several points on the plate—beginning, middle and end of the shutter
travel—the complete characteristics of the shutter can be determined.

_Interrupted light type of shutter tester._ For the study of uniformity
of shutter action alone the apparatus shown in Fig. 23 may be employed.
_A_ is a high intensity light source, such as an arc or a gas filled
tungsten lamp. _L_ is a convex lens, focussing an image of the light
source on a small aperture in the screen _E_. _D_ is a sector disc
which, driven by the motor _M_, interrupts the transmitted light with a
frequency determined by the number of openings of the sector and by the
speed of rotation, which must be measured by a tachometer. The light
diverging from the aperture in _E_ falls upon the shutter _S_, which for
this test is reduced to a narrow slit of one millimeter or less. Passing
through the shutter opening the light falls upon the photographic plate
_P_. The principle is simple: If the light is uninterrupted, the plate
_P_ is exposed at all points; due to the interruptions, a series of
parallel lines of photographic action result, and their distance apart
gives a measure of the speed of the shutter at any chosen point in its
travel. A performance curve of the French Klopcic shutter is shown in
Fig. 24. The variation in speed lies over a range of two to one. So
serious is this defect in these shutters that diaframs are sometimes
inserted in the French cameras to cut off part of the light from the
lens on the most exposed end of the plate. This expedient produces
uniformity of photographic action, but does not overcome the movement of
the image, which is one of the chief faults of excessive exposure.

[Illustration:

  FIG. 24.—Performance of Klopcic shutter.]

[Illustration:

  FIG. 25.—Optical system of shutter tester for Air Service, U. S. Army.]

A more complete apparatus, adapted both to absolute speed determinations
and to the study of uniformity of action, is that worked out and used in
the United States Air Service (Fig. 25). At _A_ is a high intensity
light source, an image of which is focussed by the lens _L__{1} upon a
slit _E_, in front of which stands a tuning fork _T_, of period 1024 or
2048 per second. The light diverging from the slit is received by a
second lens, _L__{2} which is arranged either to focus the slit image
upon the shutter curtain or to render the rays parallel, so that an
entire camera may be inserted. In the latter case the camera lens
_L__{3} serves to focus the slit image on the curtain _C_. After passing
through the curtain aperture the light is focussed by the lens _L__{4}
on the rotatable drum _D_, which carries a strip of sensitive film.

The operation of testing a shutter consists in focussing the slit image
on the portion of the shutter whose performance is required, striking
the tuning fork to set it vibrating, rotating the drum rapidly and
setting off the shutter. There is thus obtained on the sensitive film an
exposed strip resembling in appearance the edge of a saw, the number of
teeth showing the time interval in vibrations of the tuning fork. Three
exposures usually give all the points necessary for a practical
knowledge of the shutter's uniformity of action. A point of some
importance, learned from numerous shutter tests, is that a focal-plane
shutter should be tested in the position in which it is to be used.
Aerial camera shutters should be tested in the horizontal position.

=Types of Focal-plane Shutters.=—A variety of means have been utilized
for securing the necessary variation in speed in focal-plane shutters.
Their success is to be measured by the actual speed range and by the
uniformity of speed attained. In aerial cameras at present in use we
find _variable tension_ of the curtain spring, the aperture being fixed;
_variable opening_ with fixed tension; _multiple curtain openings_ with
fixed spring tension; and combinations of two or all of these methods of
speed control. The problem of covering the aperture during the operation
of winding up or setting the shutter has led to further elaborations of
shutter mechanism. These take the form of _lens or shutter flaps_,
_auxiliary curtains_, and shutters of the _self-capping_ type. Shutters
embodying all these features are briefly described below.

=Representative Shutters.=—The Folmer variable tension shutter is used
on the United States Air Service hand-held and hand-operated plate
camera and on some of the film cameras. It consists of a fixed aperture
curtain wound on a curtain roller in which the spring can be set to
various tensions, numbered 1 to 10. The range of speeds attainable is at
best about three to one, or from 1/100 to 1/300 second, considerably
shorter than the range indicated as desirable. Its uniformity of travel
is variable with the tension, as shown by representative performance
curves in Fig. 30. Lacking any self-capping feature the shutter is
provided either with an auxiliary curtain, or in the hand-held camera
with flaps in front of the lens, opened by the exposing lever before the
curtain is released (Fig. 39). This shutter is made a removable unit in
the 18 × 24 centimeter hand-operated camera, but is built into the
hand-held and film cameras.

[Illustration:

  FIG. 26.—Removable four-slit shutter of German (Ica) camera, showing
    flaps.]

_The Ica shutter_ used on the standard German aerial cameras is a good
example of the multiple slit curtain (Fig. 26). Four fixed aperture
slits are provided, with a single tension, the openings roughly in the
ratio 1, ½, ¼, ⅛, which when the spring tension is properly adjusted
give exposures of 1/90, 1/180, 1/375, 1/750 second. To pass from one
exposure time to another the setting milled head is wound up to
successively higher steps or else exposed one or more times without
resetting, depending on the direction it is desired to go. Capping
during setting, or during exposure, in order to change the opening, is
provided for by a pair of flaps on the shutter unit, which open into the
camera body. The mechanical work on these shutters is of excellent
quality, the curtain running with exceptional smoothness. Provision is
made for adjusting the tension until the marked speeds are attained;
this is presumably done in a repair laboratory to which the shutter only
need be sent, as it is a removable unit. Tests made on one of these
shutters wound to its highest tension are shown in Fig. 30. The marked
speeds are not attained, and there is considerable lack of uniformity
from start to finish of the travel.

_L camera variable-aperture shutter._ The shutter of the L type camera
(Fig. 27) is representative of one of the most primitive methods of
varying aperture. The two jaws of the slit are held together by a long
cord passing completely around the aperture, fastened permanently at one
end and attached at its other end by a sliding clasp or saddle. As this
saddle is forced in one direction the slit is closed, in the opposite
direction the cord becomes slack, and after the shutter is released once
or twice the slit assumes a wider opening. A chronic trouble is the
breaking of the cords. Its opening can be changed only after the plate
magazine is removed.

[Illustration:

  FIG. 27.—“L” type camera showing open negative magazines and shutter
    mechanism.]

_U. S. Air Service variable-aperture shutter._ This shutter is
incorporated in the American deRam and in other late American cameras
(Fig. 28). Its characteristic feature is the introduction of an idler,
whose distance from the main curtain roller can be varied. Tapes whereby
the following curtain is attached to the spring roller pass over this
idler, and by changing its position the aperture or distance between the
two curtain elements is altered over a large range. Tests of this
shutter are shown in Fig. 30. A speed of 1/50 second is provided for by
a slit width of five centimeters, and the highest speed is fixed only by
the practical limit of approach of the jaws. Experiment shows great
uniformity of rate of travel to be attainable by combining careful
choice of spring length and tension with good workmanship in the
mechanical features. Variable-aperture fixed-tension shutters have a
definite advantage over the variable-tension type in that they can
utilize for all speeds that tension which gives uniform action. The
capping feature of this shutter is provided in the American deRam by
flaps, in the automatic film camera by an auxiliary curtain. The shutter
is removable in the deRam, but built into the other camera.

[Illustration:

  FIG. 28.—Variable aperture curtain developed in U. S. Air Service, and
    used in American deRam, and “K” type automatic film cameras.]

_The Klopcic_ variable-tension, variable-aperture, self-capping shutter
is an example of an attempt to meet all shutter requirements with an
entirely self-contained mechanism. It is shown diagrammatically in Fig.
29. Tapes _G__{1}, _G__{2} are used to connect the following curtain _B_
directly to the spring roller _T_, at a fixed distance, while the
leading curtain, _A_, may be slid along the tapes by small friction
buckles, _C__{1}, _C__{2}, auxiliary springs _R__{1}, _R__{2} serving to
keep it taut in any position. When the shutter is being set the buckles
are arrested against stops while the winding-up continues for what is to
be the following half of the curtain in exposing. When released the
curtain moves across with an aperture fixed by the point of setting of
the buckle stops. At the end of the travel the buckles are arrested by
other stops, while the following portion of the curtain continues its
travel to the end. On re-winding, therefore, the aperture is closed.
Variable tension as well as variable aperture is provided, although
little used. In the French cameras a lens flap is also inserted behind
the lens, but this is not needed if the self-capping feature functions
properly. On the hand cameras this flap is said to be necessary in order
to prevent a curious kind of accident: if the camera is held on the
knee, pointing upward, an image of the sun may be formed on the curtain
and burn a hole through it.

[Illustration:

  FIG. 29.—Mechanism of Klopcic variable aperture self-capping shutter.]

The performance of the French shutter in respect to uniformity has
already been shown in Fig. 24. It leaves very much to be desired.
Besides non-uniformity of action during its travel it exhibits another
common defect of variable-tension shutters, namely, the curtain must be
released several times after a change of tension before the new speed is
established (Fig. 30, tensions 5 and 5´).

[Illustration:

  FIG. 30.—Performances of various shutters used on aerial cameras.
    Speeds expressed in reciprocals of fractional parts of one second.]

The French shutter as made for the deMaria cameras is a removable unit.
The small size (13 × 18 cm.) sets by the straight pull of a projecting
pin, the larger (18 × 24 cm.) by winding up a milled head. The former is
the more convenient motion for an aerial camera. Care must be taken with
either type that the motion of setting is not stopped when the first
resistance is encountered; this occurs when the tape buckles strike
their stop and the slit begins to open.




                               CHAPTER VI
                      PLATE-HOLDERS AND MAGAZINES


In the earlier days of airplane photography the ordinary plate-holder or
double dark slide was used to some extent, but it is ill-suited to the
purpose because of the considerable time and attention required for its
operation. It has nevertheless the merit of adding little to the length
of the camera, and it works in any position. For these reasons it has
remained in occasional use for the taking of oblique views with long
focus cameras in a cramped fuselage.

Next in order of progress rank the simple box magazines, for holding a
dozen, eighteen or twenty-four plates, as used in the English C, E, and
L type cameras. These are little more than boxes with sliding lids which
when open permit the introduction or removal of the plates. Figs. 45 and
46 illustrate the magazine of this type as made for the English C and E
cameras. It is constructed of wood, grooved to fit tracks on the camera,
and is furnished with a sliding door or lid hinged in the middle to fold
down out of the way when open. The eighteen plates are carried in metal
sheaths, both to provide opaque screens between them, and to protect
them from injury in the mechanism of the camera. Fig. 27 shows the
all-metal magazine made for the American model L camera. This differs
from the English in material of construction, plate capacity (24 instead
of 18) and manner of operating the slide, which is built up of three
thicknesses of phosphor bronze and draws out through metal guides bent
into semicircular form. A snap catch holds this slide at either end of
its travel. The leather strap introduced in the American model for
carrying and handling is a distinct improvement. These magazines contain
no springs or other mechanism, as the cameras with which they are used
depend upon the action of gravity for emptying the upper (feeding)
magazine, and filling the lower (receiving) one.

[Illustration:

  FIG. 31.—Aerial hand camera (U. S. type A-2).]

Next in order of complexity may be ranked the _bag magazine_ (Figs. 31
and 44). In this the exposed plate is pulled out of the magazine proper
by a metal slide or rod into a leather bag. The rod is then pushed back,
the plate in its metal sheath is grasped through the leather bag, lifted
to the back of the magazine, and forced in behind the other plates. The
number of plates exposed is indicated either by numbers on the backs of
the sheaths, visible through a red glazed opening in the back, or else
by a counter actuated by the metal slide rod. Usually twelve are carried
in a magazine. For aerial work the common design of this magazine as
used for ground work must be modified by providing extra large easily
grasped hooks both on the draw rod and on the dark slide, which must be
drawn before making the first exposure and replaced after the last. The
small rings and grips of the standard commercial magazine are almost
impossible to handle through heavy gloves.

The next type of magazine is represented by three designs, the _Gaumont_
and _deMaria_, used very generally by the French during the war, and the
_Ernemann_, used almost universally in the German air service (Figs. 32,
40 and 42). In all of these the operation of plate changing is the same:
the end of the magazine is pulled out and thrust back, a more simple
operation than the bag manipulation just described. The internal
workings are different according to size. In the smaller French
magazines (13 × 18 cm.) the camera is first pointed upward, all the
plates are drawn out except the one to be changed, and this, with the
aid of springs, drops to the bottom, after which the other plates push
back over it. The plates pull out in the direction of their long
dimension. In the larger French magazine (18 × 24 cm.) only the exposed
plate pulls out. The pull is in the direction of the shorter dimension
of the plate, which is lifted up by heavy springs and slides back over
the top of the pile. In the Ernemann magazine only six plates are
carried, which there is good reason to believe represent the maximum
feasible number, judging by the reports of jambs and breakages in the
twelve-plate French magazines. In all of these magazines laminated wood
slides pull out and in at each operation, and while satisfactory if made
and operated in one climate, experience indicates that if made in
America and sent abroad swelling of the wood may be expected to prevent
their successful operation.

[Illustration:

  FIG. 32.—Various plate magazines used on aerial cameras.]

Alternative forms of magazine, somewhat more practical from the
standpoint of manufacture and export, are several designs embodying _two
compartments_ (Fig. 32). In the most simple of these the plates are
moved, immediately before or after exposing, from the unexposed to the
exposed side. Illustrative of this type are the Folmer designs, in which
the to-and-fro motion is imparted by a rack geared to a pinion actuated
either by a lever, in the hand camera, or by the power drive, in the
automatic design (Figs. 33 and 53). Another illustration is afforded by
the Piserini and Mondini magazine, in which the operation of changing is
performed by a back-and-forth motion of a hand-grip, which also sets the
camera shutter (Fig. 47).

[Illustration:

  FIG. 33.—U. S. Air Service hand camera, with two-compartment magazine.]

[Illustration:

  FIG. 34.—Film type hand camera.]

In these magazines the center of gravity changes as the exposed plates
are moved over, and only half the inside space is occupied with plates.
These objections are overcome in the Chassel form, where both
compartments are always full. Transfer of the bottom exposed plate from
one compartment to the other is compensated for by the simultaneous
shift of the top plate in the receiving compartment, to the feeding
side. In a modification of this idea by Ruttan the exposing position is
when the plates are half-way through the shifting process, whereby the
magazine may be symmetrically mounted on the camera body.

[Illustration:

  FIG. 35.—Apparatus for straightening plate sheaths.]

Other more complicated magazines have been designed, some of which are
shown in the diagrammatic _ensembles_ of Figs. 32 and 48. In the
Jacquelin, the main body of plates is raised while the bottom (exposed)
plate is folded against the side. The main body of plates then drops
back to place, the exposed plate is carried on upward and folds down on
the back of the pile. The Bellieni magazine system uses three, a central
feeding one and two below for receiving, one on each side of the camera
body. These were made solely for attachment to captured German cameras.
In the Fournieux magazine the plates are carried in an interior rotating
box. The plate to be exposed is dropped off the front of the pile, down
to the focal plane, and after exposure is picked up and placed at the
back of the pile, which has turned over in the meanwhile. The deRam
rotating magazine is described in connection with the camera of which it
is an essential part (Fig. 52).

[Illustration:

  FIG. 36.—Training plane equipped for photography, showing “L” camera
    in floor mount and magazine rack forward of the observer.]

For the protection of the plates during their manipulation, and in the
camera, all plate magazines thus far developed carry them in thin _metal
sheaths_. These add greatly both to the weight and to the time necessary
to handle the plates, but no means have as yet been found for dispensing
with them. Fig. 35 shows a representative sheath or septum, as used in
the L camera. On three sides the edge is bent up and turned over,
forming a ledge for the plate to press against. The fourth side is left
open for inserting the plate, which is then held in by a small upward
projecting lip, and kept close against the ledges by narrow springs at
the sides. To insert or remove the plate the projecting lip is
depressed, either by springing the sheath by pressure from the sides or
by using an appropriate tool.

_Care of sheaths._ Unless systematically taken care of, plate sheaths
become bent or dented. They are then a menace to camera operation,
catching or jamming in the plate changing process, breaking plates and
damaging camera mechanisms. In order to maintain them flat and true,
steel forms are necessary on which the sheaths may be hammered to shape
with a mallet (Fig. 35).

_Magazine racks._ Reconnaissance and mapping call for a number of
exposures much greater than the capacity of one 12, 18, or 24 plate
magazine. Additional magazines must therefore be carried. These should
be in racks convenient to the observer (Fig. 36), securely held yet
capable of quick removal and insertion. In the rack designed to carry
two of the metal magazines for the American L Camera, the magazines
slide into loose grooves formed by a metal lip. They are prevented from
slipping out by a spring catch, past which they slide when inserted but
which is instantly thrown aside by pressure of the thumb as the hand
grasps the magazine handle for removal.




                              CHAPTER VII
                   HAND-HELD CAMERAS FOR AERIAL WORK


=Field of Use.=—The first cameras to be used for aerial photography were
hand-held ones of ordinary commercial types. Indeed the idea is still
prevalent that to obtain aerial photographs the aviator merely leans
over the side with the folding pocket camera of the department store
show window and presses the button. But the Great War had not lasted
long before the ordinary bellows focussing hand camera was replaced by
the rigid-body fixed-focus form, equipped with handles or pistol grip
for better holding in the high wind made by the plane's progress through
the air. Even this phase of aerial photography was comparatively
short-lived. The need for cameras of great focal length, and the need
for apparatus demanding the minimum of the pilot's or observer's
attention, both tended to relegate hand-held cameras to second place, so
that they were comparatively little used in the later periods of the
war.

Yet for certain purposes they have great value. They can be used in any
plane for taking oblique views, and for taking verticals, in any plane
in which an opening for unobstructed view can be made in the floor of
the observer's cockpit. They can be quickly pointed in any desired
direction, thus reducing to a minimum the necessary maneuvering of the
plane, a real advantage when under attack by “Archies” or in working
under adverse weather conditions.

For peace-time mapping work the hand-held camera, when equipped with
spirit-levels on top, and when worked by a skilful operator, possesses
some advantages over anything short of an automatically stabilized
camera. For experimental testing of plates, filters and various
accessories, the ready accessibility of all its parts makes the
hand-held camera the easiest and most satisfactory of instruments.

The limitations of the hand-held camera lie in its necessary restriction
to small plate sizes and short focal lengths, and in the fact that it
must occupy the entire attention of the observer while pictures are
being taken—the latter a serious objection only in war-time.

=Essential Characteristics.=—In addition to the general requirements as
to lens, shutter and magazine, common to all aerial cameras, the hand
camera must meet the special problems introduced by holding in the
hands, especially over the top of the plane's cockpit. An exceptionally
good system of handles or grips must be provided whereby the camera can
be pointed when pictures are taken, and held while plates are being
changed and the shutter set. The weight and balance of the camera must
be correct within narrow limits; the wind resistance must be as small as
possible; the shutter release must be arranged so as to give no jerk or
tilt to the camera in exposing.

As to the method of holding the camera, a favorite at first among
military men was the pistol grip, with a trigger shutter release (Fig.
37). Because of the size and weight of the camera the pistol grip alone
was an inadequate means of support and additional handles on the side or
bottom had to be provided for the left hand. Small (8 × 12 cm.) pistol
grip cameras were used to some extent by the Germans (Fig. 42), and a
number of 4 × 5 inch experimental cameras of this type were built for
the American Air Service (Fig. 37). But the grasp obtained with such a
design is not so good as is obtained with handles on each side or with
flat straps to go over the hands. The camera balances best with the
handles in the plane of the center of gravity. As to weight, no set
rules are laid down, but experience has shown that a fairly heavy
camera—as heavy as is convenient to handle—will hold steadier than a
light one. The American 4 × 5 inch cameras described below weigh with
their magazines in the neighborhood of twelve pounds.

[Illustration:

  FIG. 37.—Pistol-grip aerial hand camera.]

=Representative Types of Hand-held Cameras.=—French and German hand-held
cameras are essentially smaller editions of their standard long-focus
cameras, and a description of them will apply to a considerable extent
to the large cameras to be discussed in a later chapter. The English and
American hand-held cameras are generally quite different in type from
the large ones, which are used attached to the plane.

[Illustration:

  FIG. 38.—Diagram of French (deMaria) 26 cm. focus hand camera, using
    13 × 18 cm. plates.]

_The French hand-held camera_ uses 13 × 18 centimeter plates, carried in
a deMaria magazine, and has a lens of 26 centimeters focus. The shutter
is the Klopcic self-capping type already described, and is removable.
The camera body, built of sheet aluminum, takes a pyramidal shape. In
Fig. 38, _A_ is the shutter release and _B_ the rectangular sight, of
which _C_ is the rear or eye sight. The complete sight may be placed
either on the top or on the bottom of the camera. At _D_ are the
handles, sloping forward from top to bottom; _E_ is a catch for holding
the magazine; _F_ is a door for reaching the back of the lens and the
lens flap; _G_ is a snap clasp for holding the front door of the camera
closed; _H_ is a ring for attaching a strap to go around the observer's
neck; _I_ is the lever which opens the flap behind the lens and releases
the focal-plane shutter; _J_ is a snap catch for holding the front door
of the camera open.

The operations with this camera are three in number. Starting
immediately after the exposure, the camera is pointed lens upward and
the plate changed by pulling the inner body of the magazine out and then
in; next the shutter is set; then the camera is pointed, and finally
exposed by a gentle pull on the exposing lever.

_The English hand-held camera_ (Fig. 186). This differs from the French
in the size of plate (4 × 5 inch), in the shape of the camera body,
which is circular, and in the type of shutter, which is fixed-tension
variable-opening. In the longer focus camera (10 to 12 inch) the shutter
is self-capping, and the aperture is controlled by a thumb-screw at the
side. In the smaller (6 inch) a lens flap is provided in front of the
lens and the shutter aperture is varied by a sliding saddle and cord.
The handles of the camera are placed vertical, instead of sloping as in
the French. The shutter is released by a thumb-actuated lever. Double
dark slides are used, as the multiple plate magazine has not found favor
in the English service.

_The German hand-held camera_ (Fig. 42). The German hand-held camera is,
like their whole series, built of canvas-covered wood, the body having
an octagonal cross-section. It is equipped with the Ica shutter and uses
the Ernemann six plate (13 × 18 cm.) magazine. The excellent system of
grips by which the camera is held and pointed is an especially
commendable feature. On the right-hand side is a handle similar to the
French type, but carefully shaped to fit the hand. The left-hand grip
consists of a long, rounded block of wood running diagonally from top to
bottom of the side, with a deep groove on the forward side for the
finger tips, while over the hand is stretched a leather strap, the whole
aim being to provide an absolutely sure and comfortable hold on the
camera during the plate changing and shutter setting operations.

[Illustration:

  FIG. 39.—Front view of U. S. aerial hand camera, showing lens flaps
    partly open, and details of tube sight.]

_United States Air Service hand cameras._ The hand camera developed for
the United States Air Service and manufactured by the Eastman Kodak Co.
is made in three models, using the bag magazine, a two-compartment
magazine, and roll film, respectively. The shutter is of the fixed (one
or two) aperture variable tension type, built into the camera. A
distinctive feature is the double lens flap, in front of the lens
actuated by the thumb pressure shutter release (Fig. 39). In the bag
magazine camera the shutter is set, as a separate operation, by a wing
handle, and a similar handle controls the tension adjustment. In the
two-compartment type (Fig. 33) the shutter wind-up is geared to the
plate changing lever, so that but one operation is necessary to prepare
the camera for exposure. In the film type (Fig. 34) a single lever
motion sets the shutter and winds up the film ready for the next
exposure. After the last exposure of all the film is wound backward on
its own (feeding) roller before removing from the camera. The film is
held flat by a closely fitting metal plate behind, and by guides at the
edges in front, an arrangement which with small sizes works fairly well
although the exquisite sharpness of focus attainable with plates is not
to be expected. The saving in weight made possible by the use of film in
place of plates in metal sheaths is about three pounds per dozen
exposures.

In all these cameras the sight—a tube with front and back cross wires—is
placed at the bottom. This position has been found the most convenient
for airplane work, as it necessitates the observer raising himself but
little above the cockpit, a matter of prime importance when the
tremendous drive of the wind is taken into account.




                              CHAPTER VIII
                   NON-AUTOMATIC AERIAL PLATE CAMERAS


The ideal of every military photographic service has been an automatic
or at least a semi-automatic camera, in order to reduce the observer's
work to a minimum. Yet as a matter of fact almost all the aerial
photography of the Great War was done with entirely hand-operated
cameras. The primary reason for this was that no entirely satisfactory
automatic cameras were developed, cameras at once simple to install and
reliable when operated. Even the propeller-drive semi-automatic L type
of the British Air Service was very commonly operated by hand, for many
of the pilots and observers regarded the propeller merely as another
part to go wrong.

Any automatic mechanism in the airplane must work well in spite of
vibration, three dimensional movements, and great range of temperature.
The requirements were well recognized when the war closed, but had not
yet been met. Careful study of the conditions and needs by competent
designers of automatic machinery may be expected to result at an early
date in reliable cameras of the automatic type, but the description
below of hand-operated cameras really covers practically all the cameras
found satisfactory in actual warfare.

=General Characteristics of Hand-operated Cameras.=—As distinguished
from the hand-held cameras the larger hand-operated cameras are
characterized by the greater focal length of their lenses, the size of
plate employed, and the manner of holding—by some form of anti-vibration
mounting attached directly to the fuselage.

Except for the early English C and E type cameras which called for 10
inch lenses and 4 × 5 inch plates, the general practice at the close of
the war by agreement between the French, English and American Air
Services, was for the use of 18 × 24 centimeter plates and for lenses
with focal lengths of approximately 25, 50 and 120 centimeters. The
English also made use of a 14 inch (35 centimeter) lens, and never made
a regular practice of anything larger than 50 centimeters. The Germans
and Italians restricted themselves to the 13 × 18 centimeter size of
plate, while a lens of 70 centimeters focal length was standardized with
the Germans, in addition to the 25, 50, and 120 centimeter.

The particular focal length was determined by the nature of the
photographic mission. Where large areas were to be covered at low
altitudes or without the demand for exquisite detail, the shorter focus
lenses suffice. The most commonly used lens in the French Service was
the 50 centimeter, while the 120 was employed when high flying was
necessary or when minute detail was required. As already mentioned, the
common practice was to keep cameras of all focal lengths available, but
the ideal at the close of the war was to have the camera nose and lens a
detachable unit, so that any focal length desired could be secured with
the same camera body.

_The standard French camera._ The hand-held form of French camera has
already been described. The cameras for larger plate sizes and longer
focus lenses differ only in the addition of a Bowden-wire distance
release for the shutter and in the use of the Gaumont magazine which
operates without the necessity of pointing the exposed side of the
magazine upward. Fig. 40 illustrates the 50 centimeter camera, and Fig.
41 the 120.

[Illustration:

  FIG. 40.—50 centimeter deMaria hand operated camera on tennis ball
    mounting.]

[Illustration:

  FIG. 41.—120 centimeter deMaria camera.]

_The German Ica cameras._ These are larger editions of the light wood
hand camera already described, but with the addition of a Bowden-wire
shutter release. The body of the larger cameras carries a distinctive
feature in the distance control of the lens diafram, worked by means of
a lever which actuates racks, pinions and connecting rods leading to the
lens. On the side of the camera body a shallow box is provided for
carrying the color filter in its bayonet joint mount to fit on the lens
(Figs. 42 and 43).

[Illustration:

  FIG. 42.—German aerial cameras.]

[Illustration:

  FIG. 43.—Diagram of German 50 centimeter camera.]

[Illustration:

  FIG. 44.—U. S. hand-operated aerial camera (type M) with 10 and 20
    inch cones.]

_The hand-operated bag-magazine camera_ of the United States Air Service
(Type M) is similar to the small hand-held camera, but differs in three
respects: a removable shutter (of the variable-tension fixed-aperture
type) embodying an auxiliary curtain for capping during the setting
operation; a Bowden-wire shutter release; and the employment of a set of
standard interchangeable cones to hold lenses of several focal lengths.
The 20 inch and 10 inch cones are shown in Fig. 44. The operation of
this camera is similar to the French standard cameras, but not so simple
because of the number of motions required in manipulating the bag. Its
chief objection for war work lies in fact in the magazine, which should
be superseded by a two-compartment or other satisfactory type of plate
changing chamber. The camera alone, with 20 inch cone, weighs
approximately 40 pounds; the loaded magazine, with its plates in metal
sheaths, 15 pounds.

[Illustration:

  FIG. 45.—English C type aerial camera.]

_The English C and E type cameras._ The C and E type cameras have now
chiefly an historic interest. They were the first used in the English
service, fixed to the fuselage, and were later used in training work in
England and in the United States. They were never built for plates
larger than 4 × 5 inch nor for lenses of more than 12 inch focus, a
limitation set by the lenses available at the time of their design.

[Illustration:

  FIG. 46.—English type “E” hand-operated plate camera.]

In several respects the mode of operation of the two types is the same.
The unexposed plates are held in a magazine lying above the camera, in
the axis of the lens (Fig. 32). After exposure the bottom plate is
carried to one side and allowed to fall by the action of gravity into
the receiving magazine. In the C type (Fig. 45) an opaque slide is drawn
between the lens and the (variable-opening) shutter during the setting
operation. During the exposure period this slide projects into a
compartment on the opposite side of the camera from the receiving
magazine, thus making the camera mechanism three plates wide. In the E
type (Fig. 46), a flap over the lens makes it possible to dispense with
the sliding screen, and reduces the camera to about the width of two
plates. In the C type the plates are changed by a handle on top of the
camera; in the E type provision is made for distance control by cords,
and for shutter release by a Bowden wire. In both cameras the operation
of plate changing also sets the shutter, a definite advance over the two
preparatory motions in the French apparatus. The C type was constructed
of wood, the E of metal.

[Illustration:

  FIG. 47.—Italian (Piserini and Mondini) two compartment magazine
    hand-operated camera.]

_Italian two-compartment magazine camera._ A camera designed by Piserini
and Mondini was used to some extent by the Italian service toward the
close of the war (Fig. 47). This has the desirable feature just noted in
the C and E cameras: the operations of plate changing and shutter
setting are performed in a single motion. Unlike those cameras, however,
the plates are changed from one compartment to another of the magazine
already described, without dependence on gravity, by an entirely
positive shifting action. The setting of the self-capping focal-plane
shutter is accomplished by a projecting finger engaging the shutter
mechanism. Cameras of this general type, built for 18 by 24 centimeter
plates, with interchangeable lens cones, removable shutters, and
preferably magazines in which the center of gravity does not shift as
the plates are changed, represent the next step in advance of the French
practice, and may indeed prove all that is necessary or desirable in
camera complexity for peace-time photography from the air.

_The standard Italian camera and similar types._ The camera (Lamperti)
which the Italian Air Service used almost exclusively during the war
exemplifies a type quite different from anything as yet described (Figs.
48 and 49). Plates to the number of twenty-four (13 × 18 cm.) are loaded
into a chamber at the top of the camera. Each plate is held in a septum
furnished with projecting lugs at one end. A lever acting through a
Bowden wire, exposes the bottom plate, which then swings downward about
these lugs as pivots, and is forced by a pair of fingers into a
compartment at the side. The between-the-lens shutter has a single speed
of 1/150 second, and variation of exposure is achieved by altering the
lens aperture.

[Illustration:

  FIG. 48.—Various plate changing devices.]

The great advantage of this camera is its simplicity, a single motion
performing all the operations. Its disadvantages are its dependence on
gravity for operation, its between-the-lens shutter, the limitation set
to the number of exposures, and the necessity for removing the whole
camera to take out the plates for developing. In actual practice the
camera has worked out well. The better light found in the Italian as
contrasted with the northern theatre of war makes the between-the-lens
shutter at high speed adequate, while the limitation to the number of
exposures has been met by carrying several complete cameras in each
plane. Because of the Bowden-wire operation these cameras need not be
accessible to the observer or pilot, so that the practice of carrying
them in single-seaters was common. Attempts at standardization of Allied
practice through the adoption of standard lens cones were, of course,
out of the question with this camera. With its limitations of shutter
efficiency and plate size it is doubtful whether it would have been
satisfactory outside the service for which it was developed.

[Illustration:

  FIG. 49.—Italian (Lamperti) single-motion plate camera, on
    anti-vibration tray.]

The limitations set by the between-the-lens shutter in this type have
been overcome in an experimental camera along similar lines made by the
Premo Works of the Eastman Kodak Company, and in the French Aubry model
(Fig. 48). These employ focal-plane shutters which swing out of the way
and are set as the exposed plate swings or drops to the receiving
chamber. The dependence on gravity in this type could doubtless be
avoided by positive finger mechanisms. If so, the resultant cameras, set
and exposed by a single motion, would acquire a highly desirable
simplicity of operation. They would have peculiar merit because of the
very short interval required between exposures—a characteristic needed
for making low stereo-oblique views. The cameras just mentioned have,
however, departed far in form from the lines of standardized practice
and have not been followed up.




                               CHAPTER IX
                  SEMI-AUTOMATIC AERIAL PLATE CAMERAS


In the hand-operated camera the limit to progress is set when the number
of operations is reduced to a minimum. In cameras using the larger sizes
of plates a reduction in the number of operations almost inevitably
results in inflicting considerable muscular labor upon the operator.
Furthermore, distance operation becomes difficult to arrange for,
because the common reliance—the Bowden wire—is unfitted for heavy loads.
Consequently, for setting the shutter and changing the plates we must
resort to some other source of power than the observer's arm. Air-driven
turbines or propellers have been used on aerial cameras, as well as
clock-work, and also electric power, the latter derived either from a
generator or from storage batteries. The relative merits of these
sources of power form the subject of a separate chapter. Mention only is
here made of the form of drive actually employed in connection with the
various cameras.

The term _semi-automatic camera_ is best used to designate that type in
which the observer (or pilot) has merely to release the shutter, after
which the mechanism performs all the operations necessary to prepare for
the next exposure. There has been some difference of opinion as to
whether it is ever advisable to go further than this with plate cameras.
The English Service holds that completely automatic exposing, in
addition to plate changing, is apt to encourage the making of many more
pictures than necessary, involving carrying an excessive weight of
plates. The French Service has rather generally favored entirely
automatic cameras in theory, although during the war practically all the
work of the French army was done by the hand-operated cameras already
described.

=The English L Type Camera.=—The L, a modification of the earlier C and
E models, differs from its predecessors chiefly in the addition of a
mechanism which when connected with a suitable source of power can be
used whenever desired for changing the plates and setting the shutter.
As in the C and E types, all unexposed plates are carried in a magazine
above the camera, while the exposed plates are shifted in a horizontal
direction to one side and fall thence to a receiving magazine.

[Illustration:

  FIG. 50.—American model, English “L” type semi-automatic camera.]

[Illustration:

  FIG. 51.—Mechanism of “L” camera.]

Fig. 50 shows the American model, which is a copy, with modifications,
of the original English design. Its weight with one loaded magazine is
about 35 pounds. Its manner of functioning may be studied from the
picture of the mechanism (Fig. 51). The part of the mechanism to the
left is inoperative during hand operation, and the large toothed wheel
is locked by the removable pin shown hanging on its chain in Fig. 50. To
change a plate and set the shutter the projecting lever (Fig. 50) is
thrown over and back. This causes a sliding tray, in which the exposed
plate rests, to travel to the right, over the receiving magazine, where
the plate is dropped. After this the tray returns to the left exposing
position. Simultaneously the shutter is wound up. Exposure is made
either by pressing down upon the plunger, or better, by using a Bowden
wire. Provision for both methods of exposing, one for the pilot and one
for the observer, is shown in Fig. 81. The shutter is the
variable-aperture type already described, provided in addition with a
tension adjustment on the back of the camera. A flap behind the lens
does the capping during the setting operation.

For power operation the camera is connected through a flexible shaft
with a wind driven propeller (Figs. 50, 83 and 84). The locking pin is
now moved over from the toothed wheel to the lever arm, so that the
rotation of the worm driving the large toothed wheel forces the lever
through its plate changing motion. To prevent repetition, a part of the
periphery of the toothed wheel is cut out, so that it stops when its
cycle is run. When the Bowden wire actuates the shutter release it
forces the toothed wheel around into engagement (aided by one spring
tooth) and so starts the cycle once more.

When connected with the air propeller the worm is rotated continuously.
Other sources of power—an electric motor, for instance—can be attached
through the same kind of flexible shaft. If an electric motor is
employed it may be run continuously or it may be operated with an
insulated sector introduced into the large toothed wheel so that the
electric circuit is broken and the motor stops until the wheel is once
more forced around by the exposing lever.

_Faults of the L camera._ The L camera was the mainstay of the English
Air Service. In fact for the last two years of the war it was
practically the only camera the English used, and they thought highly of
it. It is, of course, subject to the limitation of small plate size and
short focus lens. It is in many ways an inconvenient camera to handle.
For instance, the upper magazine cannot be closed or removed until all
the plates are passed through. Its dependence upon gravity for the plate
changing operation is a fundamental weakness, responsible for its
frequent tendency to jam in the air. Experience made the English
observers very expert in relieving these jams. Sometimes they would turn
the propeller backward (mounting it in an accessible position to provide
for this contingency), sometimes they would shake or thump the camera.
But while these makeshifts would serve to secure pictures—the chief
object, of course, of the photographic service—they can scarcely be said
to render the camera satisfactory.

Moreover, the propeller drive has not been universally approved, as it
furnishes an additional mechanism to make trouble. Since it is not
feasible to change from power to hand operation while in the air, the
camera is put out of commission whenever the propeller or shaft is
disabled. Bowden-wire controls for both plate changing lever and shutter
release were common in the British service, which considered the extra
operation or the extra muscular exertion unimportant when compared with
the greater assurance of reliable action.

=The English LB and BM Cameras.=—During the closing months of the war an
improved L type camera was constructed, the LB. This differs from the L
in a number of detail changes, dictated by experience. The shutter is
now made removable and self-capping. Pivoted lugs are provided to hold
the exposed plate horizontal until the very instant it drops, in an
effort to prevent jams caused by the plates piling up at an angle in the
receiving magazine. The chief addition, however, is the provision of
several interchangeable cones and cylinders, for carrying lenses of
focal lengths from 4 to 20 inches. Fig. 95 shows the LB with 20 inch
lens cylinder mounted on a bell crank support in the camera bay of an
English plane.

The BM camera is but a larger edition of the LB, for 18 × 24 centimeter
plates. It also carries several interchangeable lens cones.

=The American model deRam camera.=—The rotary changing box devised by
Lieutenant deRam of the French army and incorporated in his entirely
automatic plate camera, has been adapted by the American Air Service to
a very successful semi-automatic camera. Fig. 52 shows the principle of
this changing box. The pile of fifty plates, each in its sheath, is
carried in a rectangular box open at top and bottom. The lower plate
next the focal-plane shutter is first exposed; the pile then rotates
about a horizontal axis through a complete turn. When the exposed plate
arrives in a vertical position it is allowed to drop off, by the opening
of cam actuated fingers, and lodges against the side of the enclosing
camera box proper. Still further along in the cycle the plate is thrown
off from its lodging place into a “scoop” on the top of the rotating
container and laid on the top of the plate pile. Meanwhile the curtain
of the focal-plane shutter winds up, at the same time that it is
depressed out of the way of the revolving plate container. Although the
plate changing operation depends on gravity, it nevertheless functions
satisfactorily up to 30 degrees from the vertical.

The shutter in this model is the variable-aperture fixed-tension type,
adjusted by pivoted idlers (Fig. 28). In the exposing position it runs
within three millimeters of the plate surface, and is therefore of high
efficiency for all openings. Capping during the operation of setting is
performed by flaps at the bottom of the camera body. Interchangeable
cones are supplied for lenses of various focal lengths.

For hand operation the changing box is turned over by means of a handle,
which rotates four times for the complete cycle (Fig. 90). For
semi-automatic operation an additional mechanism is provided on the side
of the rectangular camera body, copied with some necessary modifications
after the L camera power drive. From the observer's standpoint the
operation of the whole camera is the same as in the L camera, with the
important exception that power operation in no way interferes with hand
operation. Indeed, the hand can help out if the power flags or fails.

[Illustration:

  FIG. 52.—Diagram of automatic plate camera movements.]

This camera is most satisfactorily driven by a 12 volt ⅒ HP electric
motor working through a flexible shaft attached to a swivel connection
at the front of the semi-automatic drive box. A change once every four
to five seconds is possible, but greater speed is apt to throw the
changing plate too violently for safety.

The chief practical objection to this camera is its bulk. Its great
height makes it impractical for many planes. Its weight of nearly a
hundred pounds is a formidable load for a plane to carry, but this is no
more and probably less than that of any other camera when taken up with
the same number of plates in magazines. The price paid for economizing
in magazine weight is that the whole camera body, excluding the lens
cone, must be carried to and from the plane for both loading and
unloading.




                               CHAPTER X
                     AUTOMATIC AERIAL PLATE CAMERAS


=General Characteristics.=—The ideal in the automatic plate camera is to
provide a mechanism which will not only change the plates and set the
shutter, as does the semi-automatic, but make the exposures as well, at
regular intervals under the control of the operator. Such a wholly
automatic camera would leave the observer entirely free for other
activities than photography and it is to meet this tactically desirable
aim that the war-time striving for automatic cameras was due.

It is obvious that the one essential difference between the automatic
and semi-automatic types lies in the self-contained exposing mechanism
with its device for the timing of the exposures. There is no difficulty
in arranging for the driving power to trip the shutter, but it is no
easy matter to design apparatus which will space the exposures equally,
and at the same time permit of a variation of the interval. It is indeed
the crux of the problem of automatic camera design to provide for the
easy and certain variation of the interval from the two or three seconds
demanded for low stereoscopic views to the minute or more that high
altitude wide angle mapping may permit. This problem is one intimately
bound up with the question of means of power drive and its regulation,
and will be treated in part in that connection. It is to be noted,
however, that there are in general two modes of exposure interval
regulation. One is by variation in the speed at which the whole camera
mechanism is driven. The other is by the mere addition to a
semi-automatic camera of a time controlled release which affects in no
way the speed of the plate changing operation. In many respects the
latter is the best way to make an automatic camera.

While the advantages of automatic cameras are great it must not be
overlooked that a camera which can only be operated automatically is of
limited usefulness. It is not suited for “spotting” at any definite
instant, as, for illustration, at the moment of explosion of a bomb. It
should, therefore, be the aim of the automatic camera designer to so
build the apparatus that it can, at will, be used semi-automatically. In
addition, to meet the contingency of any break-down in the source of
power, the camera should be capable of hand operation, as in the case of
the American semi-automatic deRam. In short, the automatic camera should
not be a separate and different type; it should merely have an
additional method of operation.

Certain desirable mechanical features of all aerial cameras have already
been enumerated. Some of these may be repeated here with the addition of
others peculiar to automatic cameras. As a general caution, mechanical
motions depending on gravity or on springs should be avoided. Movements
adversely affected by low temperatures (20 to 30 degrees below zero,
Centigrade), are unsuitable. All adjustments called for in the air must
be operable by distance controls whose parts are large, rugged, and not
dependent on sound or delicate touch for their correct setting. The
center of gravity of the camera should not change during operation
(important in connection with the problem of suspension). The camera
should work in the oblique as well as in the vertical position. The
power required for operation must not exceed that available on the
plane. Electrical apparatus, for instance, should not demand more than
100 watts.

Any devices which diminish the weight of the camera are particularly
desirable in automatic plate cameras, because of the large number of
exposures which such cameras encourage. For instance, if the plates
could be handled without placing them in metal sheaths we should gain a
substantial reduction in weight (the sheaths weigh nearly as much as the
plates) as well as in the time necessary for handling.

=The Brock Automatic Plate Camera.=—This camera is somewhat similar to
the same designer's film camera, both in shape, in size, and in its
employment of a heavy spring motor for the driving power. It uses 4 × 5
inch plates, and carries a 10 to 12 inch lens.

The plate-changing operation is unique. As shown diagrammatically in
Fig. 52, the unexposed plates are carried in a magazine on top of the
camera, the exposed ones in a magazine inserted in the body of the
camera, directly below the unexposed magazine. The bottom plate of the
exposed pile drops into a sliding frame and is carried along the top of
the camera to the exposing position. After exposure, the plate is
carried back and drops into the receiving magazine. In order for the
plate to fall only the proper distance at each stage of the cycle,
special plate sheaths are necessary. These are cut away to form edge
patterns which clear or engage control fingers so as to ride or fall
through the sliding frame as required.

The camera is entirely automatic in operation. Regulation of the
exposure interval is by a special form of variable length escapement
controlled through a Bowden wire, in a manner parallel to that in the
Brock film camera, described elsewhere. These plate cameras were never
produced in quantity.

=Folmer 13 × 18 Centimeter Automatic Camera.=—This camera, also never
manufactured in quantity, is shown in Fig. 53, and a sketch of its
manner of operation is included in the _ensemble_ of automatic camera
diagrams (Fig. 52). Its most distinctive feature is perhaps the use of a
two compartment magazine. This is similar in form to the one already
described in connection with the hand-held cameras, but larger, to hold
eighteen 13 × 18 centimeter plates. The unexposed plates are placed in
one compartment, and after exposure are shifted to the other. The
transfer is effected by the motion of a rack, which is part of the
magazine and which is driven by a toothed pinion, also part of the
magazine, which in turn engages in a toothed wheel projecting upward
from the camera body. This toothed wheel is turned first in one
direction and then in the other by an arrangement of gears and levers
driven by the source of power, which as shown in Fig. 53 is a wind
turbine connected through a flexible shaft. Operation is either
automatic or semi-automatic as desired, and the camera can be put
through its cycle by hand if necessary.

[Illustration:

  FIG. 53.—Folmer 13 × 18 centimeter automatic and semi-automatic plate
    camera.]

[Illustration:

  FIG. 54.—French model deRam automatic plate camera.]

As with several other designs, the completion of the working model of
this camera occurred after agreements had been reached by the Allies, as
to plate size, standard lens cones, and other features, not easily
incorporated in it, thus making manufacture inadvisable. The validity of
the design for peace-time work is, of course, not affected by this fact.

=The deRam Camera.=—The only completely automatic plate camera actually
produced commercially before the end of hostilities was the French model
deRam (Fig. 54). Its plate-changing action has already been described in
connection with the American semi-automatic model (Figs. 52, 90 and 91).
It differs from the American model in the shutter, which is of the
self-capping variety, carried on a rising and falling frame; and in the
exposing mechanism. The latter embodies a clutch whose point of
attachment to a uniformly rotating disc in the camera is governed
through a Bowden wire, whereby the interval between the plate-changing
operation and the shutter release is varied. The intervals are indicated
by figures on the dial to which the observer's end of the Bowden wire is
attached. The source of power for the camera is a constant speed
propeller. Complete semi-automatic operation is not possible, as an
interval of 1 to 2 seconds elapses between the time a single exposure is
called for and its occurrence. No arrangement is provided for hand
operation.

It will be noted that while this camera is a true automatic apparatus it
does not meet even a majority of the requirements listed above as found
desirable by experience. There exists a great opportunity for designing
and developing an entirely satisfactory automatic plate camera—provided
it is agreed that anything more than semi-automatic operation is ever
advisable for plates.




                               CHAPTER XI
                          AERIAL FILM CAMERAS


The weight of the glass and the sheaths in the plate camera forms its
most serious drawback. This weight must be reckoned at least three
quarters of a pound for each 18 × 24 centimeter plate. Consequently,
with the use of these large plates, and with the demands for ever
increasing numbers of pictures to be taken on long reconnaissance
flights, a serious conflict arises between the weight of the
photographic equipment and the carrying capacity of the plane. Among
plate cameras probably the most economical in weight is the deRam. It
carries fifty 18 × 24 centimeter plates, and has a total weight of
approximately 100 pounds. An advance to 100 or 200 plates—not feasible
in the deRam construction—even if we assume the lightest possible
magazines, would bring the weight of camera and plates to 150 or 200
pounds, which would be detrimental to the balance and would seriously
infringe on the fuel carrying capacity and ceiling of any ordinary
reconnaissance plane.

Early and persistent attention was therefore paid to the possibilities
of celluloid film in rolls, as used so widely in hand cameras and in
moving picture work. The two great advantages of film would be its
practically negligible weight (approximately one-tenth that of plates,
not including sheaths) and its small bulk, which would permit the
greatest freedom in the development of entirely automatic cameras to
make exposures by the hundreds instead of by the dozens. Certain
disadvantages were foreseen at the outset: the difficulty of holding the
film flat and immune from vibration in the larger sizes; the difficulty
of quickly developing and drying large rolls; the question whether as
good speed or color sensitiveness could be obtained in sensitive
emulsions when flowed on a celluloid base as on glass. Early trials
revealed a further problem to solve: how to eliminate the discharge of
static electricity occurring at high altitudes, especially when the
weather is cold.

As far as camera construction is concerned the chief problems are to
hold the film flat, and to eliminate static.

=Methods of Holding Film Flat.=—Several means have been proposed and
used for holding the film flat. Disregarding mere pressure guides at the
side, which are suitable only for small area films (up to 4 × 5 inch),
the successful means have taken three forms: _pressure of a glass
plate_, _pressure of the shutter curtain_, and _suction_. A glass
pressure plate can be used in either of two ways; the film may be in
continuous contact with it or may be pressed against its surface only at
the moment of exposure. The advantage of this first method lies solely
in its mechanical simplicity; its disadvantage in the likelihood of
scratches or pressure markings on the film. Where a glass plate is used
there is always the chance of a dust or dirt film accumulating, or of
the condensation of moisture, to impair the quality of the negative.
There is, moreover, an inevitable loss of light (about 10%), together
with some slight distortion, due to the bending of the marginal oblique
rays through the thickness of the glass. In cases where a filter would
normally be employed, the loss of light is minimized by using yellow
glass for the plate, so that it serves for filter and film holder as
well.

_Pressure of the shutter curtain_ is utilized in the Duchatellier film
camera by furnishing the edges of the curtain aperture with heavy velvet
strips, whose light and gentle pressure during the passage of the
shutter holds the film against a metal back. In many ways this is the
simplest film-holding device; it occasions no loss of light, and needs
no mechanical movements or external accessories, such as are called for
in the suction devices next described. There is always danger of
markings on the film, if the velvet is not of the right thickness and
softness, and the operation and speed control of the shutter are
necessarily complicated by the additional frictional load.

_Suction_ of the film against a perforated back plate has been found a
very successful means of securing flatness. Suction at the moment of
exposure may be produced by the action of a bellows, which has been
compressed beforehand by the camera-driving mechanism. Continuous
suction can be produced either by a continuously driven pump, or by a
Venturi tube placed outside the fuselage. The Venturi tube (Fig. 55)
consists of a pipe built up of two cones, placed vertex to vertex, to
form a constriction. When air is forced through this at high velocity
suction is produced in a small diameter tube taken off at the
constriction. A suction of two centimeters of mercury, acting through
holes about one centimeter equidistant from each other in the back
plate, has been found adequate to hold flat a film 18 × 24 centimeters.

One merit of suction applied only at the moment of exposure is that the
film-driving mechanism does not have to work against the drag of the
suction. Continuous suction, on the other hand, gives a longer
opportunity for flattening out kinks in the celluloid, and easily
permits movement of the film during the exposure, either for the purpose
of permitting a longer exposure or for the purpose of preventing
distortion due to the focal-plane shutter. A disadvantage of continuous
suction is the production of minute scratches on the celluloid surface
as it drags over the suction plate. These are ordinarily too small to
cause trouble, but may show up when printing is done in an enlarging
camera.

[Illustration:

  FIG. 55.—Venturi tube on side of plane.]

=Static discharges= are produced by the friction of the celluloid
against the pressure back or other surfaces with which it comes into
contact. They show in the developed film as branching tree-like streaks
(Fig. 56) and in cold dry weather may be numerous enough to ruin a
picture. The discharges are noticeably less frequent with film coated on
the back with gelatine (“N.C.”), but the extra gelatine surface is
extremely undesirable. When handled by developing machines, as large
rolls must be, this back gelatine surface becomes scratched and bruised
in a serious manner. Plain unbacked film is much to be preferred if the
static can be obviated.

To avoid static, it is necessary to provide for the immediate
dissipation of all acquired electrical charges. Experiments made by the
United States Air Service have shown that nothing is so good as rather
rough cloth, thoroughly impregnated with graphite, held in close contact
with the celluloid during as great a portion of its travel as possible.
In the United States Air Service film camera which uses suction through
a perforated back plate, the plate has been covered with thin graphited
cloth, and similar cloth sheets are pressed against the film rolls by
sheets of spring metal (Fig. 65). In cameras with this equipment no
trouble has been experienced with static.

[Illustration:

  FIG. 56.—Print from film camera negative, showing static discharge,
    and (upper left-hand corner) record of altitude and compass
    direction made by Williamson film camera auxiliary lens (Fig. 58).]

=Representative Film Cameras.=—_The English F type_ (Williamson). This
is one of the earliest cameras designed for film, as is indicated by the
nature of the power drive, which presupposes that the camera is to be
carried on the outside of the fuselage. Its essential features are shown
in Figs. 57 and 58. It consists of a rectangular box with a cone at the
front on which is mounted a propeller, intended to be rotated by the
wind made by the motion of the plane. This drives, through a governor
controlled friction clutch, a train of gears which draws the (5 × 4
inch) film across the focal plane, sets and exposes the shutter at
regular intervals.

[Illustration:

  FIG. 57.—English type “F” (Williamson) automatic film camera.]

Above the camera, supported on a tripod, are a compass and altimeter,
both recording on a single dial, illuminated from below by the light
reflected from a circular white disc painted on top of the camera. An
image of the dial is thrown on a corner of the film by a lens, whose
shutter is actuated in synchronism with the main focal-plane shutter. No
special means are provided for holding the film flat. Special film with
perforated edges is used.

The camera was designed for mapping work on the Mesopotamian and other
fronts where no maps at all existed.

_The Duchatellier_ camera is essentially a film magazine to fit on the
standard French deMaria camera bodies, of the 18 × 24 centimeter size.
In its simplest form it embodies a shutter (the regular focal-plane
shutter of the camera being removed) and a film-moving mechanism, both
actuated by a single motion of the hand. Automatic and semi-automatic
operation are accomplished by an auxiliary mechanism to which Bowden
wires from the hand lever are attached. The motive power is an air
propeller. Variation of speed is obtained by changing the point of
contact of a roller on a friction disc, the disc being directly
connected to the propeller shaft, the roller to the camera drive shaft.

[Illustration:

  FIG. 58.—Interior of type “F” camera, showing lens for photographing
    compass and altitude readings.]

The most distinctive features of the Duchatellier camera is its use of
the focal-plane shutter to hold the film flat during the exposure. As
already explained, this is accomplished by pressure, velvet strips on
the shutter edges keeping the film close against the back plate. The
return of the shutter curtain to the “set” position is accomplished by
locking it to the film by perforating points, so that it is pulled
across as the film is wound. This introduces between each pair of
pictures a strip of tremendous over exposure, as wide as the curtain
opening. A fixed-aperture variable-tension shutter is used. The magazine
carries a roll of film long enough for 200 exposures, feeding the long
way of the picture. When film needs to be changed in the air, this is
done by changing the entire magazine, including its shutter.

[Illustration:

  FIG. 59.—G. E. M. automatic film camera.]

_The G. E. M. camera_ (Fig. 59) is a very light self-contained
clock-work-drive camera taking 36 pictures six inches square. The film
is unrolled from a small-diameter feeding roller on to a large-diameter
receiving roller to which the driving mechanism is attached. By this
means approximately equal spacing of pictures on the film is assured.
The film is held flat by continuous contact with a glass plate, which is
made of yellow glass, so that it serves at the same time as a color
filter. The lens—of 8 to 12 inch focus—is equipped with a single speed
between-the-lens shutter. The operation of the camera is entirely
automatic. The interval between pictures is controlled by varying the
clock-work speed, through a lever on the outside of the camera box.
Protection of the camera from vibration is sought by supporting it on
four spring cushions mounted on a solid frame, to which the camera is
held by spiral springs attached to its sides.

[Illustration:

  FIG. 60.—Brock automatic film camera.]

_The Brock Film camera_ (Fig. 60) is an entirely automatic, very compact
self-contained camera, taking one hundred 4 × 5 inch pictures. The
motive power is clock-work, regulated in speed by an escapement
controlled by a flexible shaft carried to a dial which may be fastened
to the instrument board or to some other convenient part of the plane.
The lens is 6, 12, or 18 inch focus. The shutter is of the
fixed-aperture variable-tension type, of long travel, and with a flap
behind the lens for covering during the setting period. None of the
special means above described for holding the film flat are provided. A
metal plate resting on the back, and a flat metal frame in front with a
4 × 5 inch aperture, are considered sufficient check on the excursions
of the small-sized film. A ball bearing double pivoted frame serves to
support the camera in a pendulous manner, permitting it to assume a
vertical position after tilting. Damping of oscillations and vibration
is arranged for by two pneumatic dash pots.

The _German film mapping camera_, shown in Fig. 61, is distinguished by
a number of special features. The size of the pictures, 6 × 24
centimeters, is unusual. It has its advantages, however. Since the short
dimension is in the line of flight, the maximum width of field covered
by the lens is utilized (Fig. 17). This of course necessitates a larger
number of exposures to complete a strip, which is perhaps an added
advantage, since the narrower the individual pictures the better the
junctions will be, especially if large overlaps are made. This proved to
be the case with captured German mosaics. Difficulty is experienced in
making overlaps on a turn (Fig. 62), but this is not a vital objection.
The shutter has a fixed aperture, narrower at the center than at the
ends, to compensate for the falling off in illumination away from the
center of the lens. No safety flap is needed because the curtain moves
in opposite directions on successive exposures, thereby also
compensating for shutter distortion, as has already been discussed.
Shutter speed is controlled by varying the tension of the actuating
spring.

[Illustration:

  FIG. 61.—German automatic film camera.]

The camera is driven by an electric motor, connected to a set of gears,
whose shifting provides for speed variation. The film is moved by rubber
rollers which are cut away for part of the circumference, allowing the
film to stand still until they bite again. A yellow glass pressure plate
holds the film during the exposure and serves as color filter also (Fig.
63). An electric heater is provided near the shutter, as in all the
later German cameras.

[Illustration:

  FIG. 62.—Method of joining and printing film from German camera.]

_United States Air Service automatic film camera_—_Type K_ (Figs. 64,
65, 92, 93, 98, 99). This is an entirely automatic camera, manufactured
by the Folmer and Schwing Division of the Eastman Kodak Co., taking 100
pictures of 18 × 24 centimeter size at one loading. As with all the
American cameras of this size, it uses the standard lens cones of any
desired focal length. The camera proper consists of a compact chamber in
which the film rollers are carried at each end forward of the focal
plane, the shutter lying between. In consequence of this arrangement the
vertical depth of the camera is the absolute minimum—short of decreasing
the length of the optical path by mirror arrangements—making it possible
to suspend the camera diagonally in the American and British planes, for
taking oblique pictures.

Flatness of the film is secured by a suction plate covered with
graphited cloth and connected with a Venturi tube. The top cover is
removed for re-loading. The shutters on the first cameras of this type
are of the variable-tension fixed-aperture design, though later ones
have the variable-aperture curtain controlled by an idler, as used in
the American deRam. An auxiliary curtain shutter serves to cap the true
shutter during setting.

The operation of the film driving mechanism is comparatively simple. It
consists of a train of gears, driven by a flexible revolving shaft
attached to some separate source of power capable of speed variation.
The action of the gears is to move the film, set the shutter and then
expose it; in the earlier cameras with the film continuously moving. In
the first cameras constructed the space between the pictures varies as
the film rolls up, due to the increasing diameter of the roll. In later
cameras the film roller is disengaged from the gears just before the
shutter is tripped, so that the film stands still during the exposure,
and is then re-engaged at a new point on a ratchet wheel governed by the
diameter of the receiving roll, whereby the pictures are equally spaced.
In all the cameras, punch marks made at the time of exposure enable the
limits of the picture to be detected in the dark room by touch.

[Illustration:

  FIG. 63.—Film winding and exposing mechanism in German film camera.]

Variable speed is arranged for in any one of several ways. For
peace-time uses a turbine attached to the side of the plane is simple
and positive, and, provided it is made of sufficient size—which is not
the case with the one shown in the Figure—will give adequate speed
regulation upon varying the aperture through which the air enters. The
Venturi tube may be carried upon the same mount, or a small rotary pump
can be attached on the same shaft. Where the high wind resistance of the
turbine is an objection the camera is driven electrically, by a motor
acting through the intermediary of a variable speed control described in
the next chapter (Fig. 68).

The camera weighs complete about forty pounds, and the film rolls about
four pounds. The latter can be changed in the air without great
difficulty provided the camera is mounted accessibly and so that the top
may be opened.




                              CHAPTER XII
                    MOTIVE POWER FOR AERIAL CAMERAS


As long as circumstances permit, hand operation still remains the most
reliable and satisfactory method of driving a camera. It is always
available, can be applied to just the amount desired, and at the time
and place needed. For instance, in a magazine of the Gaumont type (Fig.
40), what is needed is the periodic application of a very considerable
force rather quickly, and while this can be done quite simply by hand,
no mechanism has even been attempted to go through this same operation
automatically. Instead, the fundamental design of automatic magazines
has been made along other lines calculated to utilize smaller forces
more steadily applied.

It must be granted, however, that for war planes, and particularly for
single seaters, cameras should be available which are capable of
operating semi-automatically or automatically. This necessarily means
the employment of artificial power, whose generation, transmission to
the camera and control as to speed present a mechanical problem of no
small difficulty.

=Available Sources of Power.=—The sources from which power may be drawn
on the plane are four, although the various combinations of these
present a large number of alternative approaches to the problem. These
sources are:

                      1. The engine of the plane.
                      2. Wind motors.
                      3. Spring motors.
                      4. Electric motors.

These may first be considered largely from the descriptive standpoint,
leaving questions of performance and efficiency for separate treatment.

Power may be derived directly from the engine through a flexible shaft,
similar to that used for the revolution counter. This source of power is
inherently the most direct and efficient, since the engine is the seat
of all the lifting and driving energy of the plane. There is no loss
through transformation into other forms of energy, such as electrical;
or by the use of more or less inefficient intermediary apparatus, such
as wind propellers. Against the direct drive of the camera from the
engine may, however, be urged that the usual distance between engine and
camera is too great for reliable mechanical connection, as by flexible
shafting. Objections also arise from the standpoint of speed. This
cannot be controlled by the camera operator; and varies over too wide a
range, as the engine changes from idling to full speed, to fit it for
automatic camera operation. The first objection may be met by that
combination of methods of power drive which consists in transmitting the
power electrically; that is, by letting the engine operate a generator
from which cables run to a motor close to the camera. This method, of
course, sacrifices efficiency, and it breaks down when the engine speed
drops below the speed necessary to generate the requisite voltage. This
defect may in turn be met by floating in storage batteries, which brings
up the whole question of electrical drive, to be treated presently.
While use of the engine for direct drive or for generating electric
current has not been adopted in the American service, it is known that
some German planes were supplied with electric current in this way.

Coming next to the wind motors, these possess one very great merit: they
utilize a motive power that is always present as long as the plane is in
motion through the air. On the other hand, the process of using the main
propeller of the plane to pull another smaller propeller through the air
appears a roundabout way to utilize the driving power of the airplane
engine. Yet on the whole it is probable that some form of propeller or
wind turbine is the simplest and most convenient device we have for the
operation of airplane auxiliaries. As long as the amount of power
required is small, such inefficiency as is inherent in its use is offset
by its convenience and reliability. An advantage of the propeller is
that its speed is almost directly proportional to that of the plane
through the air, a desirable feature in automatic cameras provided the
proportionality is under control. Yet it is just in this matter of
varying the speed at will that the propeller presents difficulties, to
be met only by additional mechanisms for gearing down or governing.
Propellers have the practical disadvantage that they present an easily
bent or broken projection to the body of the plane (Figs. 83 and 84).
The strength of small propellers for operating auxiliaries is never so
much in question with reference to their resistance to whirling and
thrust of air as it is to their ability to withstand the inevitable
knocks and careless handling that will fall to their lot. The propeller
bracket is just what the pilot is looking for to scrape the mud off his
boots before climbing in.

The wind turbine has the advantage over the propeller that its speed can
be varied rather simply by exposing more or less of its face to the
wind. A turbine fitted with an adjustable aperture for admitting the
wind is shown in Fig. 64, in connection with the type K automatic film
camera. The turbine has the advantage of being compact and lying close
against the body of the plane. In the form figured, altogether too much
head resistance is offered—just as much for low as for high speeds—but
with proper design this need not be the case. It is, moreover, quite too
small to give the needed speed regulation, as it only begins to operate
near its full opening.

[Illustration:

  FIG. 64.—U. S. Type “K” (Folmer) automatic film camera, with wind
    turbine and Venturi tube.]

[Illustration:

  FIG. 65.—Type “K” camera, open, showing suction plate.]

Spring motors have the very real advantage that by their use the camera
can be made entirely self contained. The simplest application of the
spring motor would be to the semi-automatic camera, where no close
regulation of speed is required. In such a camera the operation of
exposing the shutter would release the spring, which would then change
the plate or film and re-set the shutter, repeating this operation as
long as the spring retained sufficient tension. Small film hand-cameras
of this type, using self-setting between-the-lens shutters, have been
designed, though not for aerial work. The possibilities of using springs
as motive power in semi-automatic cameras have not apparently been
seriously considered.

When a spring motor is used for automatic camera operation it at once
becomes necessary to add to the motor an elaborate clock mechanism for
controlling and regulating its speed of action. Springs are much better
fitted for giving power by quick release of their tension than by slow
release, and the necessary clock mechanisms for their regulation become
very heavy, as well as complicated and delicate, when they are made
large enough to do any real work. For their repair they require the
services of clock makers rather than the usual more available kind of
mechanic.

Coming next to electric motors, we meet with a source of power of very
great flexibility both in its derivation and in its application. If a
source of electric current is already provided for heating and lighting
as it is on the fully equipped military plane, and if it has sufficient
capacity to handle the camera, its use is rather clearly indicated,
irrespective of how efficiently or by what method it is produced.
Especially is this the case, from the standpoint of economy and
simplicity, if a propeller-driven generator is the source of current,
and the alternative power drive is an additional propeller for the
camera. If, on the other hand, the camera must have its own source of
electric power, the advantages and disadvantages must be closely
scrutinized. In this case either a generator must be provided, or resort
be made to storage batteries, or a combination of the two.

Ruling out a special propeller-driven generator, we are left with either
the generator driven from the engine or the storage battery. Inasmuch as
storage batteries are practically indispensable with generators, in
order to maintain the voltage constant at all speeds, it is on the whole
advisable to rely upon batteries alone. An advantage of their use is
that the power plant is entirely within the plane: All projections such
as propellers are avoided. Another merit is that the power is drawn upon
only as needed. Against storage batteries is their weight, the need for
frequent charging, and their loss of efficiency at low temperatures—a
loss so serious with those of the Edison form as to preclude their use.

When once the source of electrical energy is decided upon, its method of
application needs to be considered. Here we meet at once the peculiar
merit of electrical energy, namely, the ease and convenience with which
it may be transmitted. All we need is a pair of wires, led to any part
of the plane by any convenient route and connected by simple binding
posts. It may with equal ease be turned on or off by merely making or
breaking a contact with a switch. For operating semi-automatic cameras
this feature may be utilized in the interest of economy, if the power is
automatically turned off as soon as the plate-changing operation is
finished. Exceptionally reliable make and break contacts are necessary
to insure the success of this latter scheme.

Two methods of transforming the supply of electrical energy into
mechanical motion are available. The first is by the use of a solenoid
and plunger. This is a device practically restricted to semi-automatic
cameras, in which the operation consists of a straight to-and-fro
motion, initiated at the will of the operator. It has been used little
if at all. The second motion is the continuous rotary one secured by the
use of an electric motor. This motion is the most practical one for the
continuous operation of any mechanism, but on the other hand requires
that the imposed load be reasonably uniform at all times through the
cycle of operations. Assuming that the camera mechanism is of this
character, the motor may be attached directly to the camera, or if it
must be so large as to cause danger by vibration, it may be connected
through a flexible shaft. This use of an electric motor is very
practical for semi-automatic cameras such as the “L” or the American
deRam, in planes supplied with a suitable source of current.

When it comes to entirely automatic cameras, where uniform and
regulatable speed is required, as in making overlapping pictures for
mapping, the electrical drive is not so convenient. The shunt-wound
motor runs at nearly constant speed, while the series-wound motor in
which the speed can be regulated by the interposition of resistance, has
nothing like a sufficient range of variation for the purpose (at least
five to one is imperative) before it fails to carry the load. Hence we
must either incorporate in the camera some mechanism for varying the
interval between exposures while the speed of the motor remains
constant, or introduce an auxiliary device to effect the required
transformation in speed. If we do use an auxiliary device the train of
apparatus, consisting of battery (or generator), motor, speed control
and camera, is altogether too long; it is apt to cause annoying delays
in connecting up in an emergency, and it offers an excessive number of
chances for break-down.

=Performance and Efficiency Data.=—The first step in deciding upon
methods of power drive, and indeed in deciding whether power drive is
feasible at all, is to assemble definite data as to the power required
to drive representative cameras. Approximate figures for some of the
cameras described in previous chapters are:

                          L camera, 26 watts,
                          deRam,    60 watts,
                          “K” film, 30 watts.

These requirements—not exceeding ⅒ horse power—are insignificant in
comparison with the total of 100 to 400 horse power available for all
purposes from the plane's engine.

_Propeller characteristics._ Data on the performance of small propellers
are somewhat meagre. However, the results of the rather extensive
researches on large ones, suitable for driving planes, may be applied,
with proper reservations, to give a fair guide to the study of the
application of small propellers for driving plane auxiliaries.

The first factor to be considered is the thrust or _head resistance_
offered by a propeller to motion through the air. This varies as the
_square of the velocity_, as the _density of the medium_, and as the
_area of the body_ projected normally to the wind, the formula being

                             _T_ = _cdaV_^2

where _T_ = thrust, _d_ = density, _a_ = area, _V_ = velocity. Data on
the L camera propeller are shown in Fig. 66, where its thrust both when
free and when loaded with the camera is given, as well as that of a
solid disc of the same diameter as the propeller. For this propeller,
which is double-bladed, and six inches in diameter, _cda_ = .000275 with
the load on. The total thrust amounts to only about three pounds when
the plane velocity is 100 miles per hour. The head resistance of the
whole plane is a matter of hundreds of pounds, so that the propeller
resistance is quite negligible.

[Illustration:

  FIG. 66.—Wind propeller data.]

The next factor is the speed of revolution of the propeller, expressed
in revolutions per minute. This varies with the design—the number of
blades, their area, and pitch. For a given design the speed of
revolution is _directly proportional to the speed of motion through the
air_, and to _the density of the air_. Representative data for the L
camera propeller are shown in Fig. 67. It will be noted that the speed
goes up to 8000 for 120 miles per hour air speed. This illustrates the
necessity for great strength to withstand centrifugal force. Propellers
should be constructed of tough material, and subjected to whirling tests
up to speeds considerably in excess of any the plane will attain in any
maneuver. At low speeds the linear relationship fails, as a critical
velocity is reached—about 3500 r. p. m. for this propeller—where it
refuses to turn.

[Illustration:

  FIG. 67.—Relation between air speed and propeller revolutions.]

The fact that the speed of the propeller depends on the density of the
air has an interesting corollary, which is that a propeller adequate at
low altitudes will fail at high ones. The density of the air varies with
altitude according to the following figures:

               At 3000 meters, 72 per cent. of sea level
                  5000 meters, 59 per cent. of sea level
                  6000 meters, 52 per cent. of sea level

If we take the r. p. m. at 90 miles per hour at sea level as 6000, then
at the above altitudes the speeds will be 4300, 3500, and 3000,
respectively. The last figure is below that for which this size of
propeller stalls with its normal load, as noted in the last paragraph.
Consequently, if flying is to be done at these altitudes a larger
propeller must be carried, which will still deliver enough power at the
lower density.

The next factor to be considered is the _power furnished by the
propeller_. As a representative figure may be quoted the performance of
the L propeller. This gives 27 watts at 3600 revolutions per minute (56
miles per hour). From this figure the performance of other propellers
may be deduced from the basic laws, which are: that the _power varies as
the density of the medium_ and as the _cube of the velocity_ (assuming
constant efficiency). Since the power delivered by the six inch diameter
L propeller is already adequate at 60 miles per hour, the necessary
dimension to function satisfactorily at 100 miles per hour would need to
be only a little more than three inches, except for the desirability of
a safety factor for high altitudes and low air densities.

The _efficiency_ of the propeller is defined by the relation—

                          power delivered by the propeller
             Efficiency = ————————————————————————————————
                          power supplied to the propeller

The denominator of this fraction is the thrust times the velocity, for
which the curves of Fig. 66 supply us data for the L propeller. Using
the figures 3600 r. p. m., 56 miles per hour, and 27 watts, we find the
efficiency to be about 50 per cent. This increases with the velocity,
with a possible upper limit of 70 to 80 per cent. Since the main
propeller of the plane is not over 80 per cent. efficient we have at
most an efficiency of 64 per cent. in using a propeller drive, as
compared with taking the power directly off the engine.

In considering the use of _spring and clock-work motors_ we meet at once
with the problem of comparing the effect on the performance of a plane
of a carried weight, as against a head resistance. The efficiency of a
spring motor is measured in terms of its weight, that of a propeller in
terms of its head resistance. The general answer to this question is
given by the relation that _a pound of dead weight is equivalent to ⅕
pound head resistance_.

In order to apply this relation to the study of spring motors for
driving cameras, data are necessary on the power delivery per pound
weight of such mechanisms. Such data are not easily accessible, largely
because clock-work has not generally been seriously considered as a
motive power for large apparatus. To arrive at an approximate figure we
may take the fact that in an 8 × 10 inch film camera designed by one of
the manufacturers who have utilized clock-work, the motor weighed 30
pounds. This is equivalent to six pounds head resistance. Now the type
K, 18 × 24 centimeter film camera is operated, even with the addition of
a friction drive speed control, by means of the L camera propeller. As
shown in Fig. 66, at 100 miles per hour the head resistance of this
propeller is still less than three pounds. Consequently, it appears that
from the efficiency standpoint the clock mechanism is quite outclassed
by the wind propeller.

Coming next to the _electric motors_, the L camera and the K are both
operated satisfactorily with a 1/20 horse power motor, weighing 6
pounds. For the deRam a ⅒ horse power motor has been adopted.

Taking up efficiency considerations, we have, if the current is supplied
by a generator from the engine, a transformation factor of 70 to 80 per
cent. from mechanical to electrical energy and a similar factor in using
a motor for the camera. When batteries are employed the matter of weight
_versus_ head resistance again arises. The batteries found most
satisfactory for operating the K and deRam cameras are of the six-cell
12 volt lead type. Their capacity is 40 ampère hours at three ampères or
36 at five ampères—more than is necessary for a single reconnaissance,
but a practical figure when economy of charging and replacement are
considered. The weight of this unit is 27 pounds. To this must be added
the weight of the motor—6 lbs.—making a total of 33 pounds, equivalent
to a head resistance of nearly 7 pounds. This is more than twice the
propeller head resistance invoked to do the same work.

These considerations of efficiency have been gone into because they are
usual in studying any engineering problem and because of the insistent
demand from the plane designer that every ounce of weight and head
resistance be saved. Actually, as already stated, the load imposed by
any method of power drive is trivial in comparison with the whole load
of the plane. There is, however, an important reservation to be made,
which applies against clock-work and batteries: This is, that while the
equivalent head resistance of any camera motive power carried as dead
weight is small, its effect on _balance_ may not be so. While the use of
a propeller need not disturb the plane's balance, the weight of the
camera alone, without any driving apparatus, is already seriously
objected to on this score. The merely mechanical superiority of the
propeller as a source of motive power is on the whole rather marked.

=Control of Camera Speed.=—In the semi-automatic camera the only control
required on the speed of the operating motor is at the upper and lower
limits. It must not go so fast as to anticipate the completion of any
steps in the cycle of camera operation, such as the fall of plates or
pawls into position, which would jam the camera. On the other hand, it
must not be so slow that pictures cannot be obtained with the requisite
overlap for maps or stereoscopic views. In the American deRam camera the
cycle of operations cannot safely be put through in less than four
seconds, a short enough interval for most purposes. It is also highly
desirable in the semi-automatic camera to have the motive power capable
of stopping completely. This saves wear and tear on both motor and
camera mechanism.

In the automatic camera an extreme range of speed is called for by the
several problems of mapping, oblique photography, and the making of
stereoscopic views. For mapping alone, the shortest likely interval may
be taken as that required for work at approximately 1000 meters
altitude, for a plane speed of 150 kilometers per hour, which demands an
interval of six seconds with a ten inch lens on a 4 × 5 inch plate. For
vertical stereos at the same altitude and speed this interval is divided
by three, and low oblique stereos need even quicker operation. Hence a
range of from 1 to 30 pictures per minute should be provided for. This
requirement is difficult to meet with any simple mechanism.

From the standpoint of simplicity in speed regulation the wind turbine
of adequate vane surface has much to recommend it. It is only necessary
to present more or less of its vane area to the wind in order to secure
a considerable range of speed. The method of doing this by a shutter
interposed in front is uneconomical, but it is probable that the design
can be so altered that more or less of the turbine is exposed beyond the
side of the plane, possibly by varying the angle, to secure the same
result without introducing useless head resistance. A serious practical
objection to the turbine lies in the large vane surface necessary to
give adequate power combined with proper speed variation. In the
automatic film camera (Type K) this area should be as much as 40 to 50
square inches.

The wind propeller does not lend itself at all well to speed variation.
It cannot be partially covered from the air stream, as can the turbine,
because of the resulting strain on its mount. A possible form of
variable speed propeller, one which, however, has not yet been
practically developed, is a propeller with controllable variable pitch.
If this could be made mechanically sound it would be well-suited for
camera operation. That such a propeller could be worked out is indicated
by the good performance of a constant speed propeller developed for
radio generators and used on the French deRam camera (Fig. 54).
Parenthetically, it maybe questioned whether a constant speed propeller
is really desirable with an airplane camera. What is required is not
exposures at a definite time interval—although most of the data are in
that form—but exposures at definite intervals with respect to the motion
of the plane, which practically means with reference to its air speed.
Rather than build a camera calculated to give exposures at intervals of
so many seconds when it is attached to a constant speed propeller, we
would do better to use a propeller which responds to the speed of the
plane, in conjunction with some form of tachometer to show the rate at
which exposures are being made. This in turn should be coördinated with
the indications of a proper camera-field indicating sight.

One solution of the problem of speed control with a propeller of
practically fixed speed, is to use a governor and slip clutch as in the
English Type F film camera (Fig. 57). Here the propeller shaft and the
camera driving axle are connected by two friction discs. That on the
camera mechanism is forced against the other by a spiral spring, whose
tension is controlled by a ball governor. If the camera speed becomes
too high the governor reduces the tension on the spiral spring and the
discs slip over each other. The point where this slipping occurs is
determined by the position of the governor as a whole, and this is
controlled by a lever on top of the camera.

Another speed control device, perhaps more positive but certainly more
complicated and wasteful of power, consists of a large flat disc, driven
by the propeller or electric motor, and from which the camera is driven
by a shaft from a smaller friction disc which may be pressed against any
point from the center to the periphery of the larger disc. The speed
range attainable in this way is limited only by the size of the large
disc. An application of this idea is shown in the speed control (Fig.
68), designed for the American Type K camera when operated on an
electric motor or on a simple propeller. The same idea is utilized in
the Duchatellier film camera, in connection with the constant speed
propeller already described.

[Illustration:

  FIG. 68.—Friction disc speed control.]

On the whole it is eminently desirable from the standpoint of power
operation that the automatic camera should embody its own means for
altering the interval between exposures, so that all the external
attachment needed is a single connection to a source of power either of
constant speed, as an electric motor, or of speed proportional to that
of the plane, as with a simple wind propeller. This makes the camera
largely independent of the nature of the power supply, whereas a camera
designed for a special variable speed device is of little use on a plane
where this is not available.

=Transmission of Power to the Camera=—It has already been pointed out
that the ease of transmission of electrical energy makes it particularly
convenient for use in a plane. All other sources of power, except
clock-work incorporated in the camera, require flexible shafting, so
that the question of bearings and connections becomes a serious one,
especially when the shaft runs continuously for long periods at very
high speeds.

The shafting found most suitable is the spirally wound form commonly
known as dental shafting. This must be encased in a smoothly fitting
sheath, flexible enough to permit of easy bends. The ends of the shaft
should be equipped with square or rectangular pins to fit into
corresponding slots in the motor and camera shafts. The ends of the
shaft casing may be fitted either to attach by bayonet joints or by
smoothly fitting screw collars. At the point of attachment to the camera
it is desirable to have some form of junction adjustable as to the
direction from which the shaft may be connected, so that it need be bent
as little as possible. A right angle bevel gear offers one means of
doing this. Bearings, such as those of the propeller, should be of the
ball variety, while heavy lubrication, such as vaseline, should be
freely used, both in the bearings and in the shaft casing.

An important feature of any power drive system should be a safety
device, so that the power will race in case of any jam or stoppage in
the camera. This will often prevent serious damage through the breakage
of some relatively weak part of the camera mechanism on which the whole
force of the driving apparatus is suddenly thrown. The “L” camera
propeller is fitted with a spring friction clutch with the idea that if
the camera refuses to operate the propeller will slip instead of
wrenching the shaft to pieces.




                              CHAPTER XIII
                           CAMERA AUXILIARIES


=Distance Controls and Indicators.=—All operations connected with the
exposing and changing of plates (except the changing of whole magazines)
should be arranged for accomplishment at a distance. Other operations,
such as changing the shutter speed or the interval between exposures in
an automatic camera, which are usually done on the ground, may sometimes
be satisfactorily left for performance at the camera. Conditions of
extreme inaccessibility may, however, make it necessary to carry even
these controls to a distance. Indicators of the number of exposures
already made, and of the readiness of the camera for the next exposure,
may be attached to the camera, but often are more profitably placed at a
distance. Distance control and indication are especially necessary if
the pilot makes the exposures—a common English practice in two seaters,
and the only recourse in single seaters.

When electric power is available, electrical distance control devices
are perhaps the simplest kind, as they transmit motive power without
displacing or jarring the camera. Solenoids suffice for the simple
pressing of releases or for counting mechanisms, while small service
motors may be utilized for operations involving more work. A standing
practical objection to electrical control lies in the necessity for
using contacts, which are apt to be uncertain under conditions that
involve vibration.

_The Bowden wire_—a wire cable carried inside a heavy non-extensible but
flexible sheath—constitutes the most satisfactory mechanical means for
transmitting straight pulls. By means of “the Bowden” a pull may be
transmitted so as to be made entirely relative to two parts of the same
body, calling forth no tendency of the body as a whole to move. Thus in
the L camera shutter release (Fig. 50), the releasing lever with its
attached counter is several feet distant from the camera. If the plate
bearing the lever and sheath end is rigidly fastened down, the pressure
exerted on moving the lever acts between the lever and the end of the
sheath. This pressure passes immediately to the other end of the sheath,
while the pull on the wire is transmitted to its farther end on the
camera. In this way the conditions at the lever are reproduced, but with
the advantage that, due to the flexible cable and sheath, any vibration
of the lever support is damped out.

Due to its stretching, there is a pretty definite limitation to the
feasible length of the Bowden wire. This length is about four feet.
Where according to English practice the pilot makes the exposure, a
considerably longer wire and sheath are called for. In this case the
effective length of the release is increased by giving the pilot a
second releasing lever, connected to the first by a rigid rod (Fig. 69).
The releasing lever, wire, and all mechanical parts of the Bowden
release should be made much stronger than would be indicated by bench
tests of the camera. In the air it is impossible to decide either by
sound or by delicacy of touch whether the mechanism has acted, so that
the observer is apt to pull much harder than necessary and to strain or
break the release if it is weak.

The Bowden wire is used in the American service only for shutter
release. In the English service it has been used for plate changing with
the L camera.

[Illustration:

  FIG. 69.—Bowden wire release in rear cockpit, with rod connected to
    similar release for the pilot.]

[Illustration:

  FIG. 70.—Bowden wire release with stop watch attached, for use in
    timing for overlaps.]

=Sights.=—In airplane photography the need for a finder or sight is
fully as great as in everyday work. A new condition, however, prevails,
for except with hand-held cameras, and even to some extent with them,
the operation of pointing the camera involves pointing the whole vehicle
that carries the camera. The pointing of airplane cameras is therefore
akin to the sighting of great guns. While the observer may perform the
actual operation of taking the picture, the responsibility for covering
the objective rests with the pilot. Teamwork counts equally with tools.
Airplane camera sights may accordingly be divided into two classes:
sights attached to the camera, for use principally with hand-held
apparatus, and sights attached to the plane, for the use of pilot, of
observer, or of both.

=Sights for Hand-held Cameras.=—The simplest form of sight attached
directly to the camera is modeled on the _gun sight_, consisting of a
forward point or bead and a rear V. This sight of course serves merely
to place the objective in the center of the plate and gives no
indication of the size of field covered. Another simple sight of rather
better type is the _tube sight_—a metal tube of approximately one inch
diameter and three inches length, carrying at each end pairs of wires
crossed at right angles. The camera is in alignment when the front and
back cross wires both exactly match on the object to be photographed.
The best way to mount the cross-wires is with one pair turned through 45
degrees with respect to the other, so that it is at once apparent which
is the front and which the rear pair (Figs. 31 and 39).

_Sights to indicate the size of the field_ are usually less needed on
hand cameras than on fixed vertical cameras. Yet certain circumstances
make them most desirable, for instance in naval work where a complete
convoy must be included on the plate. A sight of this kind can be made
up of two wire or stamped metal rectangles, a large one in front and a
smaller one behind, of such relative sizes and separations that the true
camera field is outlined when the eye is placed in position to see the
two rectangles just cover each other. The dimensions should be so chosen
that the correct position of the eye is approximately its natural
location with respect to the camera when this is held in the hands in
the plane. It is usual to provide the rectangular sights with
cross-wires to indicate the center of the field. Alternative rear sights
are simple beads or peep-holes—the use of the bead assuming that the
camera is held at about the right distance from the eye for the
rectangle to indicate the field. The peep-sight is not a desirable form,
as it is hard to hold the camera as near the face as is necessary. These
various types of rectangle sights are well illustrated in the cameras
shown in Figs. 38, 40 and 186. They are all made so as to fold down flat
on the camera and to snap quickly open when needed. The springs to
support the sights must be fairly strong, and the surface presented to
the wind as small as possible. Wire frames give very little from the
pressure of the wind, but flat metal frames are apt to be bent back.

The position of the sight on the camera is important. If the observer
can stand, or if he sits up well above the edge of the cockpit, the
conventional position of the sight on a pistol, namely, on top, is
unobjectionable. But if the observer sits very low, as he usually does,
then the sight should be on the bottom of the camera, thereby avoiding
any need for the observer to raise his head unduly into the slip stream.
Similarly, if the camera is used over the side for verticals, as it is
in flying boats, a sight on the top is impractical, since it requires
the observer to lean out dangerously far (Fig. 185).

=Sights Attached to the Plane.=—Any of the sights just described can be
attached to cameras fixed in the plane, but they would be useless in the
positions ordinarily occupied by the camera. It has therefore become
common practice to attach the camera sight to some accessible part of
the plane. The most primitive method of sighting is merely to look
downward over the side—a method in general use to the very end of the
Great War. One step in advance of this is to mark a large inverted V on
the side, with its vertex at a point where the observer can place his
eye and so see the fore and aft extension of the field of view covered
by the camera. This kind of sight was common on the French “photo”
planes. On some of the English planes the tube sight was carried on the
outside of the cockpit. Any of the sights described can be carried on
the inside of the fuselage, provided a hole is cut in the floor. For
satisfactory sighting a hole in the floor is really necessary, as it
enables the terrain on both sides of the vertical to be seen. One
drawback to the simple hole, however, is that it cannot be made large
enough to show the whole field from the ordinary height of the
observer's eye, thus forcing him to bring his head down near the floor.
This difficulty is gotten over in a very beautiful way by the use of the
_negative lens sight_ shown diagrammatically in Fig. 71.

[Illustration:

  FIG. 71.—Diagram of negative lens sight.]

Let _F_{1}_ be the distance at which the edge of the hole (or a
rectangle marked on the lens) appears the size of the camera field (if
the hole is the size of the plate, _F_{1}_ is the focal length of the
camera lens). Let _F_{2}_ be the distance from the floor to the
observer's eye. What is desired is a concave lens which will diverge the
rays from their normal meeting point at _F_{1}_ to a new meeting point,
_F_{2}_. The focal length of lens required is given at once by the
simple lens formula—

                           1         1       1
                        ——————— - ——————— = ———
                        _F_{1}_   _F_{2}_   _F_

Thus if _F_{1}_ is 12 inches, and _F_{2}_ is 36 inches, _F_ will be 18
inches. The lens is to be marked with a rectangle showing the shape and
size of the camera field, and a central mark such as a cross. An upper
rectangle, or a bead, or a pair of cross wires a few inches below the
lens, may be used for the other sight. For precision work the sight
above or below the lens should be adjustable in position, especially
where the camera suspension permits the camera to be adjusted for the
angle of incidence of the plane.

A negative lens sight should be placed in the observer's cockpit, if he
takes the pictures, and also in the forward cockpit, so that the pilot
may be accurately guided in his part of the task. In addition, it is
advisable to place a negative lens well forward in the pilot's cockpit,
to enable him to see the country some distance ahead. The lenses should
be planoconcave with the flat side upward; otherwise, all the loose dirt
in the airplane settles in the middle of the concave depression. A
negative lens sight in a metal frame forming a completely self-contained
unit ready for mounting in the plane is shown in Figs. 72 and 73.

=Devices for Recording Data on Plates.=—_Numbering devices._ The number
of the camera is impressed on negatives taken with the American L camera
through the agency of a small transparent corner of celluloid. It would
be entirely possible to incorporate a rotating disc which should turn by
the operation of plate changing and carry a series of numbers, so that
each exposure could be numbered serially. Numbering of individual plates
is more commonly done by holes, notches, or even numerals, in the turned
over portion of the sheaths, which are then recorded photographically
when a picture is taken (Fig. 75). The chief objection to this method is
the difficulty of keeping the sheaths together in sets, especially as
individual ones become damaged or lost. In practice there is also danger
of the sheaths being carelessly loaded in wrong order.

[Illustration:

  FIG. 72.—Negative lens and mount, viewed from above.]

The more ambitious idea of recording on the plate all the information
given by the instrument board of the plane occurs independently and
spontaneously to all aerial photographic map makers. These ideas vary
from attempts to photograph the actual instrument board on every plate—a
difficult task indeed with the instruments and camera placed as they are
in the ordinary plane—to the incorporation of compass, altimeter, and
inclinometer in the camera itself.

Figure 58 shows the plan adopted in the English F type film mapping
camera already described, for photographing a compass and an altimeter
on the film. Here the combined compass and altimeter dial is above the
camera, and is mounted in a cell with a glass bottom. Below it is a lens
focussing the needles and compass points on the plane of the film. The
light for photography is furnished by a diffusely reflecting white
surface on top of the camera, illuminated by the sky. (The camera was
carried outboard.) In Fig. 56 is shown a picture with the compass image
impressed upon it.

[Illustration:

  FIG. 73.—Negative lens and mount, side view.]

Figure 74 shows a type of inclination indicator found in some captured
German cameras. It consists essentially of two small pendulums or
plumb-bobs; one to indicate lateral, the other longitudinal inclination,
arranged to be photographed in silhouette on the plate, as shown in the
lower part of the diagram and in the print from a captured negative
(Fig. 75).

[Illustration:

  FIG. 74.—Diagram of inclinometer used in some German cameras.]

[Illustration:

  FIG. 75.—Photograph made with German camera, showing inclinometer
    record, four points for locating diameters and center of plate, and
    (upper right-hand corner) number of the plate sheath.]

Both these devices suffer from the deficiencies of the instruments they
photograph. The compass and the inclinometer, as already mentioned in
the discussion of airplane instruments, only behave normally in
straight-away flying, failing to indicate correctly when the plane is
subject to accelerations in any direction. In general all attempts to
record directional data in the camera are of little promise, unless
either the instruments or the camera are automatically held level by
some gyroscopic device. If the instruments are so controlled, rather
elaborate means for photographing them are necessary. If the camera is
stabilized, the inclinometers are unnecessary, and the compass behaves
rationally.

Another scheme for indicating inclinations, which is not subject to the
above objections, is to photograph the horizon either on a separate film
or on the same sensitive surface, simultaneously with the principal
exposure. The difficulty here is the practical one that it is only
feasible in localities of great atmospheric clearness. Ordinarily,
especially anywhere near the sea-coast, the horizon is too rarely seen
to be a reliable mark (Fig. 4). It is possible, however, that this
objection could be overcome by the use of specially red sensitive plates
and suitable color filters, as discussed in the chapter on “Filters.”
The method would in any case be useless in mountainous country.

The difficulties discussed with reference to direction indicating
instruments of course do not hold with the altimeter. Ordinarily,
though, the altitude changes slowly enough to permit of sufficiently
accurate records being made by pencil and pad. For high precision map
making a photographic record of altimeter readings has a legitimate
claim. As we have seen, a small altimeter is incorporated in the English
F camera, but the bulk which a really precision altimeter would assume
would be a bar to its use in this way. A time or serial number record on
the plate or film, synchronized with a similar record on the film of an
auxiliary camera which photographs the altimeter and other instruments,
may be the simplest way to preserve the majority of the desired data.

=Devices for Heating the Camera.=—Parts of the camera mechanism which
depend on the uniformity of action of springs or upon adequate
lubrication are susceptible to change with variation of temperature. At
high altitudes low temperatures are met which may freeze ordinary
machine oils or may cause springs to seriously alter their tension, even
to break. To meet this difficulty, and probably also to dispel the
occasional condensation of moisture on the optical parts, the German
cameras are equipped with an electrical heating coil placed just below
the shutter, and arranged to connect with the general heating and
lighting current of the plane. Two contacts are ordinarily provided, for
offsetting the effects of temperatures of -15 and -30 degrees
centigrade. An additional function of this heating coil is perhaps to
maintain the sensitiveness of the plates or film.




                                  III
          THE SUSPENSION AND INSTALLATION OF AIRPLANE CAMERAS




                              CHAPTER XIV
     THEORY AND EXPERIMENTAL STUDY OF METHODS OF CAMERA SUSPENSION


=General Theory.=—In addition to the limitation of exposure set by the
ground speed of the plane another limitation is set by the _vibration_
of the camera. This may be caused either by the motor, or by the elastic
reactions of the plane members to the strains of flight. Unlike the
movement of the image due to the simple motion of the plane, movements
due to vibration may be eliminated by proper anti-vibrational mounting
of the camera.

The effect of vibration may show as an indistinctness of the whole
image—this is its only effect with a between-the-lens shutter—or as a
band or bands of indistinctness parallel to the curtain opening (Fig.
76). These are due to shocks or short period vibrations during the
passage of the focal-plane shutter.

The obvious remedy for vibration troubles is to mount the camera on some
elastic, heavily damping support, like sponge rubber or metal springs.
Such a mounting should, however, be designed on sound principles derived
from a proper analysis of the nature and effect of the possible motions
of the camera. Otherwise, the vibrational disturbances may be increased
rather than diminished by the camera mount. Such an analysis, based
merely on general mechanical principles, shows that all motions of the
camera are resolvable into _six_. These are three _translational_
motions, namely, two at right angles in one plane such as the
horizontal, and one in the plane at right angles to this (vertical); and
three _rotational_ motions, one about each of the above directions of
translational motion as an axis (Fig. 77).

[Illustration:

  FIG. 76.—Captured German photograph, showing zones of poor definition
    due to vibration during passage of focal plane shutter aperture.]

Brief consideration will show that only the latter—the rotational
motions—are of any importance when the small displacements due to
vibration are in question. To illustrate the negligible effect of
vibrations which merely move the camera parallel to itself in any
direction it is only necessary to imagine the camera moved parallel to
the ground through a large distance, such as 10 centimeters. Now 10
centimeters motion of the camera at 3000 meters elevation means, with a
25 centimeter camera lens,

                      .25          1
                      ———— × 10 = ———— centimeter
                      3000        1200

motion on the plate, which would be only a tenth the distance separable
by a good lens. If we reduce this motion to the small fraction of a
centimeter which vibration would actually produce, it is evident that
such vibration is of absolutely no importance. Similarly, if we imagine
the camera, under the same conditions, moved vertically with reference
to the ground by ten centimeters, the scale of the picture would merely
be changed by 1/12000 or by 1/1000 centimeter on a 12 centimeter plate,
again quite negligible.

When we consider motions of rotation, however, the case is quite
different. If the camera is mounted so that the effect of any vibration
is to rotate it around a horizontal axis, this is exactly equivalent to
rotating the beam of light from the lens so that it sweeps across the
plate. Thus a millimeter displacement of the lens of the camera with the
plate remaining fixed gives approximately a millimeter motion of the
image. Consequently, a rotation producing only a fraction of a
millimeter's relative motion of lens and plate during the period the
curtain aperture is over a given point would cause fatal blurring—and
the visible vibration of plane longerons and cross members is easily of
half millimeter amplitude or more. Reduced to angular units it is easily
shown that a rotation of one degree per second—which is quite slow as
plane oscillations go—is beyond the limits of toleration. Translational
motions of large amplitude may be allowed, but the mounting of the
camera must not permit these translations to be at all different for
different parts of the camera.

[Illustration:

  FIG. 77.—Diagram showing possible motions of the airplane camera:
    three of translation and three of rotation, and their combinations.]

The proper way to eliminate vibrational effects is to devise a mounting
that will transmit only the translational shocks or that will transform
the rotational ones into translations. Platforms pivoted and
cross-linked so as to be free to move only parallel to themselves
(described in the next chapter) represent one attempt to reach this
result. Quite the simplest and most scientific form of mounting to
achieve this end is to support the camera solely _in the plane of the
center of gravity_. The principle here involved is easily grasped if we
note that when we jar a camera supported above or below its center of
gravity, the effect is to start the camera vibrating with the center of
gravity oscillating pendulum-like about the point of support. The closer
the center of gravity to the center of support, the smaller the moment
of the body about the latter point.

=Experimental Study of Methods of Camera Support.=—Conclusive evidence
as to the merits of any system of camera mounting can be obtained only
under conditions that eliminate the effect of other variables which may
be equally efficacious in diminishing the effects of vibration, but
which have only limited application. Very brief exposures—1/500 second
and less—will, for instance, result in good pictures with almost any
condition of vibration. Hence a sharp picture offers no proof of the
merits of a camera mounting unless it is known that the exposure was no
shorter than the limit set by the ground speed of the plane. In fact it
may be said that the chief object of studying methods of camera
suspension is to increase the allowable exposure to a maximum, thus
lengthening the working hours and multiplying the useful working days
for aerial photography.

The most satisfactory method of test yet developed is to fly over a
light or a group of lights on the ground with the camera shutter open.
In the first use of this method, which originated in the English
Service, such flights were made at night, but later it was found that
good results could be got by placing the lights in a forest and making
the tests when the sun was fairly low. One of the group of lights must
be periodically interrupted, at a known rate, to furnish the time
intervals.

[Illustration:

  FIG. 78.—Tests of camera mounting, made by flying over a bright light
    against a dark background. (_a_) Rigid fastening on side of plane;
    (_b_) held in the hand, inexperienced observer; (_c_) held in the
    hand, experienced observer; (_d_) camera mounted at center of
    gravity on gimbals bedded in sponge rubber.]

Some characteristic “trails” obtained by this method of test are shown
in Fig. 78. The first trail is that given by a camera rigidly fastened
to the fuselage. The second and third show hand camera trails, made by
an inexperienced and by an experienced observer, respectively. They show
by comparison with the other figures that the human body is an excellent
block to vibration, but in unskilled hands a poor check to rapid erratic
(probably rotational) motions of the camera. The fourth is the trail
given by a camera supported by gimbals bedded in sponge rubber
accurately in the plane of the camera's center of gravity. Other trails
are shown in the next chapter in connection with the description of
practical camera mountings. Clearly the best suspension is that giving
the smallest amplitude of displacement during the interval of time
covered by an average exposure. It is, in fact, possible to determine
from these trails the permissible exposure for any assumed permissible
blurring of the image. The rigid mounting trail indicates very bad
conditions, calling for literally instantaneous exposures. The center of
gravity trail, at the other extreme, shows practically no limitation of
exposure in so far as vibration is concerned, thus bearing out the
theoretical conditions above discussed. An interesting conclusion from
these experiments is that a rapidly running motor gives less harmful
vibration than a slow one, although in the war it was a common practice
to throttle the motor before exposing. As might be expected, the greater
the number of cylinders, the shorter the period and the smaller the
amplitude of the vibration.

=Pendular Camera Supports.=—The design of the camera support may be
approached from a different standpoint, namely, with the aim of carrying
the camera so that it will tend to hang always vertical. In mapping this
is of fundamental importance. It is, indeed, a question whether aerial
mapping will ever be worthy of ranking as a precision method unless the
camera can be mounted so that its pictures are taken in the horizontal,
undistorted position.

The simplest way to hold the camera vertical is to mount it on gimbals,
with its center of gravity below the point of support. When so mounted
the camera swings as a pendulum. Delicacy of response to variation of
level is obtained by leaving a considerable distance between the center
of gravity and the center of support. Oscillation about the vertical
position is to be prevented by some system of dash pots or other
damping. A suspension of this kind is furnished with the Brock film
camera (Fig. 60).

It will be seen at once that the relation of center of gravity to center
of support called for here is in direct contradiction to the
requirements for eliminating vibration. Either one requirement or the
other must be sacrificed, or else a compromise made in which neither
delicate response to inclination of the plane nor fully satisfactory
freedom from vibration is attained. This is a very serious objection to
the pendular support. But the really vital objection to the pendular
support is that it performs its function only very partially. It is
entirely satisfactory only under conditions of steady flying, as in a
uniform climb or glide, with the plane tail or nose heavy, or in flying
with one wing down. As soon as we introduce any acceleration, as in
making a turn, the camera follows the plane and not the earth.

It is true that mapping photography is done from a plane flying as level
as possible, and that except under bad air conditions it holds its
course with very little turning, if handled by a skilled pilot.
Nevertheless, a surprisingly small deviation from straight flying causes
quite serious variations from the vertical. It is of interest to
calculate how large may be the horizontal accelerations that accompany
swervings from a straight course which one might think insignificant.
For instance, consider the horizontal acceleration due to a turn having
a radius of a kilometer when the plane is moving at 100 kilometers per
hour. If _a_ is the acceleration, _v_ the velocity of the plane, and _r_
the radius, we have from elementary dynamics that

                                    _v_^2
                              _a_ = —————
                                     _r_

Substituting the values chosen, we have—

                            100,000^2         meters
                    _a_ = ————————————— = .77 ——————
                          3600^2 × 1000       sec^2

The acceleration of gravity being 9.80 meters/sec^2 we have that the
ratio of the horizontal acceleration to the vertical is

                              .77
                              ———— = .078
                              9.80

This is the tangent of the angle of deviation from the vertical, from
which the angle turns out to be about 4½ degrees, a very considerable
error, rapidly multiplied as the speed of the plane is increased. It is,
indeed, open to question whether the average deviations from the
vertical are not apt to be less with the camera rigidly fixed to the
plane, if guided by a skilled pilot who will hold the ship level at the
expense of “skidding” the slight turns he must make to hold his
direction.

=Gyroscopic Mountings.=—The ideal support for the aerial camera will
undoubtedly be one embodying gyroscopic control of the camera's
direction. By proper utilization of the principles of the gyroscope it
is to be expected that not only can the camera be maintained vertical,
but it may be supported anti-vibrationally as well. At the present time
the problem of gyroscopic control is in the experimental stage, so that
only the elements of the problem and the possible modes of solution can
be laid out.

The gyroscope consists essentially of a heavy ring or disc rotating at a
high speed on an axis free to point in any direction (Fig. 79). If
mounted so that the axes of the supporting gimbals pass through the
center of gravity of the rotating disc, the result is a _neutral_
gyroscope. Its characteristic is that its axis maintains its _direction
fixed_, but this fixity is with respect to space and not with respect to
the gravitational vertical. Consequently, as the earth revolves the
inclination of the gyroscopic axis changes with respect to the earth. In
latitude 45° this change is approximately a degree in five minutes.
Furthermore, the action of friction in the supports, which can never be
entirely eliminated, also acts to slowly alter the direction of the
gyroscopic axis. Therefore, unless some _erector_ is applied even the
gyroscope will not perform the task required of it.

[Illustration:

  FIG. 79.—Diagram of simple gyroscope.]

Before discussing possible forms of erectors it may be noted in general,
first, that these must depend upon gravity; second, that such being the
case, they must respond to the resultant of gravity and any
acceleration, that is, to the _apparent_ or _pseudo-gravity_. As already
seen, this pseudo-gravity, during a turn, is exactly what limits the
usefulness of the pendular support, and necessitates recourse to the
gyroscope. The problem thus becomes one of making an erector-gyroscope
combination which will respond to true gravity and not to
pseudo-gravity.

In general this problem would be insoluble, since there is no difference
in the nature of the acceleration of gravity and that due to centrifugal
force. A way out is offered, however, by the fact that true gravity acts
continuously and at a small angle to the axis of the gyro, while the
components which cause the pseudo-gravity are of short duration, liable
to rapid changes of direction, and, on a turn, act at a large angle.
What we require, therefore, is an erector which will respond slowly but
surely to the _average_ acceleration, which is downward, but too
sluggishly to be affected by the shorter period accelerations due to
turns or rolls. Slowness of response is a matter of the erecting forces
being small and of the mass and angular velocity of the gyro disc being
large. The success of the compromise called for depends on the relative
times taken for the gyroscope to tilt seriously from the true vertical,
due to the causes above mentioned, and for the average turn or roll.
Fortunately the former is a matter of minutes, the latter of seconds or
at the worst of fractions of a minute. More than this, since the roll or
turn is apt to be of much greater angle than any normal deviation of the
gyroscopic axis from the vertical in the same time, we are offered the
possibility of some device for filtering out the deviations which alone
are to effect the erector. For instance, by shunting the restoring force
whenever it is called upon to act through more than a predetermined
small angle.

As to the method of erecting the gyroscope, its characteristic property
must be kept in mind. This is that the axis does not tilt under an
applied force in the direction it would if the gyro were not rotating,
but around an axis at right angles to that of the applied couple. Thus
in Fig. 79, if a weight is attached as shown, the disc does not incline
downward toward the weight, around the axis _Y_, _Y´_, but _precesses_
about the vertical axis Z, Z´. Some means is therefore needed to
translate the pull which any gravitational control, such as a freely
swinging pendulum, would give, into a pull with _at least a component_
at a finite angle to this.

In the Gray stabilizer several metal balls are slowly rotated in a tray
above the center of gravity of the gyroscope. Specially shaped grooves
or compartments limit the freedom of motion of these balls so that when
the gyro is inclined the balls travel at different distances from the
center on the ascending and descending sides. By this scheme a couple is
produced about the axis through the center and the low point of the
disc, which tilts the apparatus to the gravitational vertical. In an
alternative form the balls are carried past the low point by their
momentum and are prevented from returning by the walls of the containing
compartment, which have meanwhile been advanced by the rotation of the
erector as a whole. The net result is to shift the center of gravity of
the system of balls in the proper direction to erect the gyro. The
rectifying action is purposely made quite slow so that the displacements
of the balls due to pseudo-gravity will be averaged out.

In a design due to Lucian, small pendulums work through electric
contacts to actuate solenoids which in turn move small weights in the
appropriate directions to give the desired tilt. Response is made fairly
quick and delicate, and pseudo-gravity, due to turns and rolls, is
rendered inoperative by the contacts breaking whenever the pendulums
swing more than three or four degrees. This can only happen if they move
too quickly for the erecting forces to act, reliance being here placed
on the characteristic differences of action in respect to time of real
and pseudo-gravitational forces.

Besides the neutral gyroscope as just considered there is the pendular
or top type, in which the center of gravity is not in the plane of the
supports. In general this type depends on a couple resulting from the
gravitational pull and the inevitable friction of the supports to slowly
tilt the axis to the gravitational vertical. This type is slower to
respond than the designs in which a definite couple in the proper
direction is provided and it reaches the true vertical only through a
circuitous path.

[Illustration:

  FIG. 80.—Diagram of camera linked to gyroscopic stabilizer.]

Three methods of controlling a camera by a gyroscope are suggested. One
is to fasten the gyroscope rigidly to the camera and mount the whole
system on gimbals. A second is to mount both camera and gyro side by
side on gimbals, linking the two so that the camera is moved parallel to
the gyro (Fig. 80). A third method is to utilize the gyro to make
electric contacts to operate motors which in turn move the camera.

Considerable weight and space are required for a gyroscope capable of
stabilizing a camera. The rotating disc should be about half the weight
of the camera, and with its mounting may be expected to double the room
required for the camera alone. Motive power for maintaining the gyro in
continuous rotation may be supplied by an air blast, or the gyro may be
made up as an induction motor—the latter necessitating an alternating
current supply.

In view of the space and weight limitations in a plane it is a question
still to be decided whether it is more economical to stabilize the
camera or to stabilize an inclinometer and photograph its indications
simultaneously with the release of the shutter which takes the aerial
picture.




                               CHAPTER XV
                       PRACTICAL CAMERA MOUNTINGS


=General Considerations.=—Camera mountings as used during the war were
far from being developed on the basis of scientific study or test. At
first the need for special supporting apparatus was not realized, and
the suspensions later in use were largely field-made affairs, often
dependent on adjustments made according to individual taste. Through
lack of accurate methods of test and of conclusive evidence on the
subject, it was quite common to find extremists who, on the one hand,
denied the efficacy of suspensions in general, and on the other ardently
supported crazily conceived supporting arrangements which accurate
comparative test show to be even worse than useless.

In the French service, despite numerous types of suspension available,
the very general practice was to lift the camera from its support and
hold it between the knees. Or else the hand was pressed on the top of
the camera during exposure, more reliance being placed on the damping
qualities of the body than on any of the rubber or spring mechanisms.

As is clearly shown by the experimental data described in the last
chapter, a correctly designed supporting device, carrying the camera
accurately in the plane of its center of gravity, accomplishes
practically perfect elimination of vibrational troubles. So important is
the use of a mount and so important is it that the mount should be
correctly dimensioned and adjusted for the camera, that an entirely
different attitude should be adopted from the prevalent one which
focuses attention on the camera and regards the mounting as a mere
auxiliary to be left more or less to chance. _The mounting should be
considered an integral part of the camera._ The man in the field should
receive camera and mount together, leaving as his only problem the
attachment of the complete camera—and—mount unit to the plane. This may
be arranged, by proper designing, to be a simple matter of rigid bolting
or strapping, requiring ingenuity perhaps but not the scientific
knowledge which is required for mounting design.

[Illustration:

  FIG. 81.—“L” camera mounted outside the fuselage. Observer using
    exposure plunger, pilot using Bowden wire release.]

=Outboard Mountings.=—In the English service the camera was first
attached to the plane outside the fuselage by a rigid frame, to which
the camera was strapped or bolted (Fig. 81). Obvious objections exist to
placing the camera in this position, such as the resistance of the wind
and the difficulty of changing magazines. However, in the earlier
English planes with their fuselages of small cross section no other
accessible place for the camera was to be found. Vibrational
disturbances with the rigid outboard mounting are quite serious, as is
so clearly indicated by the trace shown in Fig. 78. Extremely short
exposures are alone possible, and a very large proportion of the
pictures are apt to be indistinct.

=Floor Mountings.=—A step in advance of the outboard mounting is to
support the camera snout in a padded conical frame on the floor of the
plane (Fig. 82). This mounting avoids the objection on the ground of
wind resistance that holds with the outboard, and has possibilities of
being worked out as an entirely satisfactory support. Yet to be
satisfactory, the point of support must lie in the plane of the center
of gravity of the camera, and the camera must be of a type that
preserves its center of gravity unchanged in position as the plates are
exposed. Unless these conditions are fully met the floor mounting gives
results little better than does the outboard.

=Cradles or Trays.=—Floor space in the cockpit being unavailable in the
battle-plane, due to duplicate controls, bomb sights, etc., the English
service was driven to the practice of carrying the camera in the
compartment or bay behind the observer. Here it was attached either to
the structural uprights or longerons, or to special uprights and
cross-pieces built into the plane to serve photographic ends. As an
intermediary between the camera and the supporting cross-pieces there
was introduced the camera _tray_ or _cradle_. This is essentially a
frame carrying sponge rubber pads into which the camera is more or less
deeply bedded. Figs. 83 and 84 show an American L camera cradle based on
the design of the English L camera tray. Thick sponge rubber pads
support the two ends of the camera top plate, and additional pads are
provided to hold the nose of the camera. Careful tests show this cradle
to be superior to the outboard mounting, but still leave much to be
desired. Its performance is better with the nose of the camera left
free.

[Illustration:

  FIG. 82.—“L” camera in floor mounting.]

[Illustration:

  FIG. 83.—“L” camera and cradle mount in skeleton DeHaviland 4
    fuselage, side view.]

=Tennis-ball Mounting.=—A very simple mount used by the French consists
of a frame enclosing the nose of the camera, and carrying four tennis
balls, on which the whole weight rests (Fig. 40). If the center of
support is in the plane of the center of gravity and if the four balls
are of uniform age and elasticity, this form of support is good. As
provided by the camera manufacturer, the tennis ball frame fits much too
far down on the camera. Another application of the tennis ball idea was
frequently made in the French service, in which the balls were close up
under the shutter housing (Fig. 85). Additional support was, however,
given to the camera nose by flexible rubber bands, the success of the
whole being largely a matter of the adjustment of the tension on the
bands.

[Illustration:

  FIG. 84.—“L” camera and cradle mount in skeleton DeHaviland 4
    fuselage, front view.]

=Parallel Motion Devices.=—A form of suspension favored by the French
consists of parallel bell cranks, rigidly linked together and held up by
springs. Mountings of this sort are illustrated in Figs. 86, 87, 88 and
96. The guiding principle is that any sort of shock will be transformed
into a straight up-and-down or side-wise motion of the camera, which is
harmless. The mounting as adapted by the English surrounds the camera
body, making the plane of support somewhere near the center of gravity.
In certain of the French suspensions employing this principle the whole
camera is hung below the bell cranks (Fig. 86), and then the nose is
restrained by heavy rubber bands. The net result is largely a matter of
adjustment.

[Illustration:

  FIG. 85.—Tennis ball suspension, assisted by elastic bands attached to
    nose of camera.]

[Illustration:

  FIG. 86.—French spring and bell crank suspension.]

[Illustration:

  FIG. 87.—U. S. hand-operated 18 × 24 centimeter plate camera on bell
    crank mount with rotating turret.]

[Illustration:

  FIG. 88.—Same camera in plate changing position.]

[Illustration:

  FIG. 89.—Tests of camera mountings: (_a_) deRam camera on
    bell-crank-and-spring mount, below the center of gravity; (_b_)
    same, at center of gravity; (_c_) type “K” film camera on universal
    mounting (Fig. 88).]

Tests on the English design made in the United States Air Service appear
to show that the chief virtue of the mounting lies in the approximation
of the point of support to the center of gravity in the English cameras.
A deRam camera supported by its cone, so that its center of gravity was
considerably above the center of support gave rather poor results (Fig.
89_a_), but when the bell cranks were attached near the center of
gravity, highly successful results were obtained (Fig. 89_b_). The
French deRam camera as ordered for the American Expeditionary Force was
fitted with a bell crank supported in this position.

Figures 90 and 91 show a bell crank mounting furnished with a rotating
turret. This was designed to facilitate the changing of magazines in the
English B M camera, which is swung around through 90 degrees from the
exposing position to bring the magazine near the observer. The camera
shown in the mounting is the American hand-operated model (type M), in
which there is the same necessity for turning in order to manipulate the
bag magazine easily. The camera is shown in both exposing and plate
changing positions. An important detail of these mounts is a _safety
catch_, which must be fastened before the plane lands, in order to
prevent the shocks of landing from producing oscillations sufficient to
throw the camera out of the mount.

=Center of Gravity Rubber Pad Supports.=—Given a camera whose center of
gravity does not change during operation, a simple and entirely adequate
anti-vibration support is furnished by a ring of sponge rubber in the
plane of the center of gravity. But if provision has to be made for
oblique views or for adjusting the camera to the vertical, something
more elaborate is necessary.

[Illustration:

  FIG. 90.—U. S. model deRam camera on anti-vibration mounting
    adjustable for the angle of incidence of the plane.]

[Illustration:

  FIG. 91.—U. S. deRam camera and mount installed in photographic
    DeHaviland 4 (Fig. 100). Viewed from above the observer's cockpit.]

[Illustration:

  FIG. 92.—U. S. type “K” film camera on universal mounting, vertical
    position.]

[Illustration:

  FIG. 93.—U. S. type “K” film camera on universal mounting, oblique
    position.]

Mountings for the American deRam and for the Air Service film camera,
embodying the results of complete study of the anti-vibration problem,
are shown in Figs. 90, 92 and 93. Trusses carrying the cameras on pivots
rest on four pads of sponge rubber which are mounted on frames correctly
spaced ready for attachment to the cross-pieces of the airplane camera
supports. In the deRam (Fig. 90) the pivots, attached to the camera
body, permit it to be leveled fore and aft, to compensate for the
inclined position of the fuselage assumed at high altitudes or in some
conditions of loading. This will sometimes amount to as much as 11 or 12
degrees, which is very serious, since one degree causes (with an angular
field of 20 degrees) about one per cent. difference of scale at the two
sides of the plate. The film camera mounting carries the camera in a
conical ring, and is pivoted not only for vertical adjustment, but for
the taking of obliques as well (Fig. 93). These mounts transmit
practically no vibration.

[Illustration:

  FIG. 94.—Tests on two types of camera mount: (_a_) Support at bottom
    of camera; (_b_) support above center of gravity.]

A caution must be noted with regard to center of gravity mountings. Any
change in the camera, in particular the substitution of a short for a
long lens cone, must be made so as to cause no alteration of the
relative positions of the center of support and the center of gravity.
Either the short cone must be weighted, or additional supporting pivots
must be provided in the plane of the new center of gravity.

=The Italian and G. E. M. Mountings.=—These mounts (Figs. 49 and 59) are
similar in that the protection from vibration is furnished by an elastic
support at the bottom of the camera. Tests show that these two cameras
give very similar results, of the unsatisfactory sort to be expected
from this kind of mounting in view of the lessons of the last chapter on
the proper point of support. Fig. 94, _a_, shows a trace given by the
Italian mount. The permissible exposure, on the criterion adopted, is
very short with either mount, about 1/200 second.

=The Brock Suspension.=—This consists of a pair of frames into which the
camera is fitted by ball bearing pivots, so that it is free to move in
any direction (Fig. 60). In order to permit gravity to control the
direction of the camera, the point of support is made considerably (ten
inches) above the center of gravity. Air dash pots are provided for
damping the swings. As already explained, the pendular method of support
is in basic contradiction to the requirements for vibration elimination.
Tests of the Brock suspension, shown in Fig. 94, _b_, indicate it to be
of low efficiency in damping out the short period vibrations which are
responsible for poor definition.




                              CHAPTER XVI
          THE INSTALLATION OF CAMERAS AND MOUNTINGS IN PLANES


=Conditions to Be Met.=—The characteristic difficulty in installing the
airplane camera is that there is no place for it. After the gasoline
supply, the armament, the wireless, the oxygen tank, the bombs, and
other necessities are taken care of there is neither space available nor
weight allowable. Where space may be found it will be inaccessible, or
accessible only through a maze of tension and control wires; or it will
be in a position where any weight will endanger the balance of the
plane. Plane design has in fact been more or less of a conflict between
the aeronautical engineer, who is designing the airplane primarily as a
machine to fly, and the armament and instrument men, who look upon it as
a platform for their apparatus. Lack of appreciation of the extreme
importance of aerial photography resulted, during a large part of the
war, in the camera installation being neglected until the plane was
supposedly entirely designed, and even in production. At that stage the
installation could be but a makeshift. Only in the later stages of the
war, when plane design became a matter of coöperation between all
concerned, were fairly convenient and satisfactory arrangements made for
the camera. Always, however, the rapid succession of new plane designs,
with various shapes of fuselage and details of structure, made camera
installation in the war plane a matter calling for the greatest
ingenuity.

The problem was met in part by constructing both cameras and mountings
in sections, to be laboriously wormed in through inadequate apertures,
in part by later structural changes in the planes, such as the
substitution of veneer rings or frames for the tension wires. In certain
cases the rear cockpit controls were omitted, thereby freeing accessible
and often adequate space for the larger cameras. Rear controls were
never used in the German planes, so that their standard practice was to
carry the camera forward of the observer. This, together with the
general restriction to the 13 × 18 centimeter size plate, made the
installation problem less difficult in the German aircraft than in the
Allied.

=Practical Solutions.=—An important feature of camera installation has
already been mentioned, but may well be repeated for emphasis. The
camera and its anti-vibration mounting should always be considered as a
unit, and should be so designed that simple bolts or straps will suffice
to fasten it in its place in the plane. Even should the spacing of the
structural parts of the plane not correspond to that anticipated by the
mounting design, the ingenuity of the man in the field may be depended
upon to make the necessary alterations or additions to the plane. The
design of the camera suspension itself cannot, however, be left to
uneducated ingenuity.

Assuming the camera and mounting supplied, the next step—a very
difficult one—is to insure uniformity in the structures to be built into
the planes for the purpose of supporting the camera mountings. With this
uniformity must, however, be combined the greatest possible flexibility
to provide for various designs of cameras.

In the English service the standard camera installation consists of
wooden uprights with cross bars athwart the plane, adjustable as to
height (Fig. 95). A distance between the cross bars of 13¼ inches has
been standardized, and all camera cradles and mountings are notched or
otherwise spaced to fit this dimension. The installation adopted in the
American planes is similar, but with a distance of 16 inches between
cross bars. These uprights and cross bars are ordinarily situated in the
bay behind the observer, but can be placed in any available space. Fig.
83 shows, in a model bay, the arrangement of uprights and cross bars in
the American DH 4, with the L camera in place in its cradle. It is just
possible to introduce camera and cradle separately from the observer's
cockpit through the tension wires, and, by uncomfortable reaching,
magazines may be changed.

[Illustration:

  FIG. 95.—“LB” camera with 20-inch lens, mounted on bell-crank
    suspension in skeleton fuselage. Stream-lined hood below to cover
    projecting end of lens cylinder. Propeller and Bowden release in
    place.]

A step in advance is made when the top tension wires and superstructure
are replaced by a rigid frame with an opening large enough to admit the
entire camera and mounting. When this is done considerably larger
cameras may be accommodated in the same sized bay, as shown in Fig. 96.
A further advance, from the standpoint of accessibility and convenience
of installation, follows when the tension wires between observer's and
camera bay are replaced by a ply-wood ring, as shown in Fig. 97. Here
the only serious limitations are those due to the vertical height of the
camera, and of course its weight.

Openings for the lens to point through are simply provided in the fabric
covered aircraft, by cutting through the canvas and stiffening the edge
of the hole by wire. Tension wires are often in the way. They may either
be disregarded, since they merely cut off a little light, or replaced in
part by metal rings, as shown in Fig. 96. In veneer covered fuselages
the hole must of course go through the wood. This may be undesirable,
since the veneer is depended on to furnish structural strength, a point
which further emphasizes the importance of the photographic requirements
being thoroughly considered while the plane is being designed.

Single seater or scout planes do not lend themselves to the insertion of
such standardized uprights and cross-pieces, because of their small size
and the common utilization of all space inside the fuselage for gasoline
tanks and control wires. Some French scouts, whose fuselages are very
wide, due to the rotary engines, have been fitted with compartments for
contemplated automatic film cameras. The most commonly used camera in
the single seater was, however, the Italian 24-plate single-motion
apparatus (Fig. 49). This camera and its carrying tray occupy very
little lateral space and have in actual practice been carried beneath
the seat or pushed up through an opening in the bottom of the fuselage
under the gasoline tank. Whatever criticism may be made of the adequacy
of the mounting, it must be said that the camera, as used, is perhaps
the most eminently practical of all developed in the war, as its use on
scouts testifies.

[Illustration:

  FIG. 96.—U. S. type “K” film camera on bell-crank mount, in camera bay
    of DeHaviland 4. Veneer frame at top of bay in place of usual
    cross-wires.]

[Illustration:

  FIG. 97.—Section of fuselage of veneer construction affording superior
    accessibility to camera.]

=Special Photographic Planes.=—As cameras grew in size, the difficulty
of installing them in planes built without regard to photographic
requirements greatly increased. Few planes could carry even the 50
centimeter focus camera obliquely without the necessity of poking its
nose through the side where it would catch wind and oil; while the 120
centimeter camera could be carried obliquely only in the fore and aft
position. Even vertical installation of the latter camera was really
feasible in but few planes; sometimes the camera was carried to the
exclusion of the observer—and, in fact, this size was never used by the
English, whose fuselages were small in cross-section.

This situation led, late in the war, to steps toward producing planes
designed primarily for photographic reconnaissance. In these the camera
would be entirely accessible, and cameras of any size could be carried
in any desired position. One scheme which properly belongs under this
heading was the provision of a special removable photographic cockpit,
for the front or nose of a twin-motored three seater. Other noses, for
bombing and heavy machine guns, were also planned, all to be
interchangeable. Since the regular photographic bay with uprights and
cross-pieces was also provided to the rear, this special photographic
ship could on occasion do two classes of work, such as long focus
spotting and short focus mapping.

The most completely worked out photographic plane was probably the model
designated P1 by the United States Air Service. This is a modified
DeHaviland 4 in which the rear controls have been removed and the
cowling raised and at the same time made squarer in cross-section. The
space formerly occupied by the rear controls provides ample room for all
types of camera. These are carried on uprights at the standard distance
apart, 16 inches, with cross-pieces adjustable as to height. The camera
space is accessible not only from the observer's cockpit, but from
above, upon folding back the metal cover. Doors at the bottom and at
each side permit not only vertical but oblique exposures. The latter are
not interfered with by the wings, as they would be in some designs of
plane if the camera occupied the same position relative to the cockpits.
Fig. 91 shows the deRam camera in place, as seen from the rear. Figs. 98
and 99 show the 18 × 24 centimeter film camera, set both for vertical
and oblique views.

Negative lenses are provided for both pilot and observer, the one for
the pilot permitting him to see from a point far ahead to directly
underneath, while the observer's is furnished with cross wires below and
etched rectangles of the camera field sizes on the upper surface.
Windows of non-breakable glass assist in sighting obliques. The
accompanying picture (Fig. 100) of the plane showing an oblique camera
in position gives an excellent idea of its appearance. Its special
features are worthy of copying in peace-time photographic aircraft.

=Installation of Auxiliaries.=—It is quite necessary that the camera
lens be protected from splashing mud and often from oil spray due to the
motor. For this purpose an easily opened and closed door is essential,
unless the camera is carried well up in the plane. An alternative,
possessing certain advantages, is to incorporate into the camera
protecting flaps operating in front of the lens, which open only when
the exposure is made. If the camera projects beyond the fuselage,
_stream lined hoods_ (Fig. 95) must be provided to protect the camera
nose with the minimum of air resistance.

[Illustration:

  FIG. 98.—20-inch focus automatic film camera mounted obliquely in
    photographic DH-4.]

[Illustration:

  FIG. 99.—20-inch focus automatic film camera mounted vertically in
    photographic DH-4.]

[Illustration:

  FIG. 100.—DeHaviland 4 re-constructed as a special photographic plane,
    showing 20-inch camera mounted for oblique photography.]

The mounting of the regular camera auxiliaries—releases, sights,
propellers, speed controls, motors—is usually a great bother, due to
lack of space and to the severe restrictions on methods of fastening.
Screws in longerons or uprights are taboo. Metal straps to go around
structural parts are the approved device, but with variations in the
size of these members, the holes, straps, bolts and nuts provided are
very apt not to fit. Changes of construction, such as that from
skeletons covered with fabric to veneer bodies, also interfere with any
standard means of attachment, and leave this, like many other problems
in war-time aerial photography, to the resourcefulness of the man in the
field.

Magazine racks must be tucked away in any available space. Under the
seat is a position frequently utilized. Especially with plates is it
desirable to carry the extra magazines in a position to interfere as
little as possible with the balance of the plane. In the DH 4 this means
that they should be carried if possible forward of the observer, even
though he must turn completely around to get and insert each magazine.




                                   IV
                   SENSITIZED MATERIALS AND CHEMICALS




                              CHAPTER XVII
     THE DISTRIBUTION OF LIGHT, SHADE AND COLOR IN THE AERIAL VIEW


The general appearance of the earth as viewed from above has already
been described and illustrated (Figs. 10 and 11). It remains to deal
with the earth's appearance in a more analytic and quantitative manner,
in order to decide upon the characteristics to be sought in our
photographic sensitive materials.

=Range of Brightness.=—The absence of great contrasts so apparent in the
view of the earth from a plane is confirmed by photometric observations.
These show that the average landscape, as seen from the air, rarely
presents a range of brightness of more than seven to one, even when seen
without the presence of veiling haze. It is to be remembered that
shadows constitute no important part of the aerial landscape. Vertical
walls in shadow, which form a substantial part of the surfaces seen by
an observer on the ground, are invisible or greatly foreshortened from
the air. Moreover, they are never contrasted against the sky, which is
photographically often the brightest part of the ordinary picture. To
the aviator's eye shadows on the ground are only of any length at early
and late daylight hours. Even at these times they cover but a small
area, since the number of high vertically projecting objects in a
representative landscape is small. Lacking shadows, the brightness range
is only that between various kinds of earth, water, and vegetation.
Chalk (from freshly dug trenches), reflected sunlight from water, or
marble buildings, furnish almost the only extensions to the brightness
scale as above given.

_Diurnal and seasonal changes._ During the winter months on the Western
Front photography from the air was only possible for two or three hours
around noon, on clear days. This calls attention to another factor of
prime importance, namely, the large variation in the intensity of
daylight during the course of the day and during the course of the year.

[Illustration:

  FIG. 101.—Variation of average daylight intensity during the day.]

Measurements showing typical variations from morning to night are
exhibited in Fig. 101, from which it appears that there is an increase
in illumination of four to five times from 8 o'clock—when it would be
considered full daylight for purely visual observation—until noon, while
there is a corresponding decrease by four o'clock. Fig. 102 shows sets
of measurements by two different authorities which give the average
intensity of daylight for each month throughout the year. From December
to July there is an increase of approximately ten times. From both sets
of data it therefore appears that—neglecting the frequent occurrence of
clouds which reduce the illumination to a half or a quarter or even
less—a variation in illumination of forty or fifty times occurs between
mid-day in summer and morning in winter. In the photography of
stationary objects on the ground this range of intensities is easily
taken care of by selection of lens stop and shutter speed. On the
airplane it is quite otherwise, because the shutter speeds called for at
the lower illuminations are much slower than the motion of the plane
will allow.

[Illustration:

  FIG. 102.—Variation of intensity of daylight through the year; two
    different sets of measurements.]

=Haze.=—At low altitudes the brightness range is substantially that
which would be obtained by photometric measurements of soil and
vegetation made at the earth's surface. At higher altitudes, especially
above 2000 meters, this brightness range is materially decreased by
atmospheric haze. The significance of this lies in the fact that for
safety from anti-aircraft guns, war-time aerial photography must be
carried out at very great elevations. Toward the end of the Great War
photographic missions traveling at from 5000 to 7000 meters were the
rule. At these heights, even in very clear weather, a veil of
bluish-white haze reduces the already small contrasts still more. Some
means for overcoming the effect of this haze becomes imperative,
therefore, in order to secure in the picture even the normal contrast of
the object.

Haze is to be sharply distinguished from clouds or fog. Clouds and fog
consist of globules of water vapor of large size, opaque to light. Haze,
on the contrary, is more opaque to some colors than to others, or is
_selective_ in its veiling effect. Its scattering action on light is
greatest in the violet and blue of the spectrum, decreasing rapidly
through the green, yellow, and red, the exact relation being that the
scattering is inversely as the fourth power of the wave-length. It is,
consequently, possible to pierce or cut haze by using yellow, orange, or
red color screens. It is this possibility which has led to the extensive
use of yellow or orange goggles for shooting and for naval lookout work.
In aerial photography the equivalent is to be found in _color filters_,
used with color sensitive (orthochromatic or panchromatic) plates, which
have been found essential for all high altitude work.

=Color.=—Visual observation from the airplane is aided in no
inconsiderable degree by the differences of _color_ that exist between
various objects of nearly the same brightness. This means of
distinguishing differences of character fails in the photographic plate,
which is color-blind; that is, it reproduces all objects as grays of
varying brightness. It is color-blind in another sense as well, in that
it evaluates colors as to brightness differently from the way the eye
does, overrating blues and violets and underrating yellows and reds.
This first kind of color-blindness is a positive disadvantage, for it
leaves available for differentiating objects only their brightness
differences. The second kind of color-blindness may on occasion actually
be an advantage. For it may happen by accident, or by design (through
the skilful use of color filters), that objects appearing nearly the
same to the eye appear different in the plate. More will be said about
this in connection with the use of filters for the detection of
camouflage.

The range of hues seen in the aerial landscape is not large. Greens
(grass and foliage) predominate, followed by browns (earth), neither
color being bright or saturated. Over towns or cities we find that grays
(roads) and redder browns (brick) are conspicuous. Blues are practically
never seen, although it is to be noted that a fair share of the
illumination of the ground is by blue sky light and that the haze itself
is bluish. Consequently, the general tone of a landscape is much bluer
than one would be apt to imagine it from consideration of the general
green and brown character of the constituent objects. A color photograph
from the air would greatly resemble a pastel in its low range of tones
and the absence of bright colors.

=The Photographic Requirements Dictated by Brightness and Color
Considerations.=—Considering only the demands made by the character of
the view presented to the airplane camera, and leaving out of account
other limitations to photographic operations in the plane, certain
requirements as to sensitized materials may be outlined. First of all,
the photographic process must not reduce, but should rather be capable
of exaggerating, the range of brightness of the object. Preferably the
seven-to-one range of the object photographed should be lengthened out
to the full range of the printing paper, which may be two to three times
this. With such an increase of range, those minute differences of
brightness are accentuated, on which the detection of many objects
depends.

Next, the plate or film must be sensitive to the portion of the spectrum
transmitted by a yellow or orange filter which will cut out the effect
of haze. This calls for orthochromatic or panchromatic plates, depending
on the depth of filter required. Next, if the objects to be photographed
differ little in brightness but are different in color composition, we
may have to rely on color filters of peculiar transmissions, capable of
translating these color differences into brightness differences. These
will, in general, call for fully color sensitive, or panchromatic
plates.

In conclusion it may be pointed out that the endeavor in ordinary
orthochromatic photography—to reproduce the visual brightness of colors
in the photographic print—has no real justification in aerial work.
Neither in respect to color values nor in respect to brightness range is
it the object of aerial photography, especially for war purposes, to
present a truthful tone reproduction. Its aim is rather the adequate
differentiation of detail, by whatever means necessary.




                             CHAPTER XVIII
               CHARACTERISTICS OF PHOTOGRAPHIC EMULSIONS


The purely photographic problem in aerial photography, as distinct from
the instrumental one, is the selection of photo sensitive materials
which will yield useful results under the conditions peculiar to
exposure from the air. After such materials have been found by extensive
field tests, it is preeminently desirable to determine their
characteristics in such terms that the kind of plate or film may
thereafter be specified and selected on the basis of purely laboratory
tests. Specification must be made in terms of the ordinary sensitometric
constants of the photographic emulsion—its speed, contrast, fog,
development factor, its color sensitiveness, its ability to render fine
detail, and its grosser physical properties such as hardness and
shrinkage.

=Sensitometry.=—The most generally used system of sensitometry is that
of Hurter and Driffield, commonly referred to as the “H & D.” By this
system, in order to determine the characteristics of a given
photographic plate, it is necessary to take a series of graduated
exposures, a standard illumination of the plate being varied in known
amount by a rapidly rotating disc cut to a series of different openings,
or by some other suitable means. The negative thus obtained is developed
in a standard developer for a definite time, at a fixed temperature, and
is then measured for transmission on a photometer. The following terms
are defined and used in plotting the results:

                          intensity of light transmitted     _I_
     Transparency = _T_ = —————————————————————————————— = ———————
                           intensity of incident light     _I__{O}

                      intensity of incident light     _I__{O}    1
     Opacity = _O_ = —————————————————————————————— = ——————— = ———
                     intensity of transmitted light     _I_     _T_

               Density = _D_ = -log_{10}_T_ = log_{10}_O_

Hurter and Driffield pointed out that a negative would give a true
representation of the differences in the light and shade of the object
if it reproduced these differences by equivalent differences in opacity.
This is equivalent to stating that if the densities are plotted against
the logarithms of the corresponding exposures, a straight line should be
obtained at 45 degrees to the axis of exposure times. If the line is at
another angle the opacities of the negative will be _proportional_ to
the brightness of the object photographed, but the _contrast_ will be
different.

A typical H & D plot is shown in Fig. 103. It will be noted that two
curves are shown. These are obtained with different developments, and
illustrate the fact that the contrast or proportionality between
exposure differences and opacity differences is a matter of time of
development. Each of these curves exhibits certain characteristics which
are common to all made in this way. There is primarily a _straight line
portion_, where opacities are proportional to illumination. This is
commonly called the region of _correct exposure_. The slope of this
straight line portion—the ratio of density/(log exposure)—is the
_development factor_, commonly denoted by “γ,” a gamma of unity denoting
exact tone rendering. Below the region of correct exposure is a “toe,”
or region of smaller contrast, called the region of _under exposure_.
Above the correct exposure region is another where the opacity
approaches constancy (afterwards decreasing or “reversing”), called the
region of _over exposure_.

The _speed_ of a plate on the H & D scale is given by the intersection
of the straight line portion of the characteristic curve when produced,
with the exposure axis. This intersection point, called the _inertia_,
is the same irrespective of the time of development, as is shown in Fig.
103. The numerical value of the speed is obtained by dividing 34 by the
inertia, when the exposure is plotted in candle-meter-seconds.

If a plate is developed until no more density and contrast can be
obtained, its development factor is then γ_{∞}, (gamma infinity), and
the larger this is the more a plate can be forced in development. If the
plate fogs in its unexposed portions this fog is measured and recorded
in density units along with the other constants. The speed of
development is represented by the _velocity constant_, commonly
symbolized by κ.

[Illustration:

  FIG. 103.—Typical characteristic curves of photographic plate.]

The length of the straight line portion determines the _latitude_ of the
plate, or the range of permissible exposures to secure a “perfect
negative.” Thus if we assume that an object has a range of brightness of
1 to 30, then a plate with a straight line characteristic extending over
a range of 1 to 120 would have a latitude of 120/30 or 4. That is, the
exposure could be as much as four times the necessary one, and still
give the same result on a sufficiently exposed print. If the latitude of
the plate is too small, the shadows will fall in the under exposure
region, the high-lights in the over exposure portion of the
characteristic curve, with consequent poor rendering of contrasts.

=Criteria of Speed.=—In airplane photography _speed_ is of paramount
importance, but great care must be exercised to insure that all the
factors are considered which can contribute toward yielding the
desirable pictorial quality in the brief exposure which alone is
possible from the moving plane. A “fast” plate on the H & D scale is not
necessarily suitable for aerial work, when we remember that accentuation
of natural contrast is desirable, particularly under hazy conditions.
For, as is shown in Fig. 104, it is a common characteristic of “fast”
plates to have comparatively small latitude and low contrast at their
maximum development.

It is to be noted that the Hurter and Driffield measure of speed is
bound up with the idea of correct tone rendering and with the use of the
straight line portion of the characteristic curve. Other criteria of
speed exist. For instance, the exposure necessary to produce a just
noticeable action (threshold value); and the exposure necessary to give
a chosen useful density in the high-lights when development is pushed to
the limit set by the growth of fog.

As has already been pointed out, correct tone rendering is not necessary
or even indicated as desirable in aerial views. It is, moreover, a
matter of experience that the majority of aerial exposures with existing
plates fall in the “under exposure” period, where contrasts with normal
development are less than in the subject. This being the case, the
problem is to select not necessarily a fast plate, by the H & D
criterion, but a plate which will develop up workable densities in the
under exposure region. A plate of medium speed will sometimes develop to
greater densities in the short exposure region, if development is
forced, than will a fast plate. The contrast in the normal exposure
region will be excessive, but this is of no significance if no exposure
falling in this region is present on the plate.

[Illustration:

  FIG. 104.—Characteristic curves of fast and slow plates, developed to
    maximum contrast.]

In addition to its capacity for developing density, the plate should
have as low a threshold as possible, thus meeting to some extent the
requirements of both the alternative criteria of speed given above. At
the same time it is true that low threshold and good density for short
exposures are not to be found in really slow plates. Consequently, while
high speed, as ordinarily understood, is undoubtedly the first
requirement, we may expect the complete specification for the best
aerial plate to be a rather complicated thing, describing the
characteristics of a workable “toe” of the curve, in terms of which
several (_e.g._, contrast and speed) are derived from another and quite
different exposure region.

=Effect of Temperature on Plate Speed.=—It has been found by Abney and
Dewar that very low temperatures materially decrease the speed of
photographic emulsions. This decrease may amount to as much as 50 per
cent. in the temperature range from 30 degrees Centigrade above zero to
30 degrees below zero, which is the range over which aerial photographic
operations will have to be carried on in war-time. This effect has not
been at all fully studied, and it is not known whether it is general or
only found in certain kinds of plates. The remedy indicated is to
provide means for heating the plates or films when low temperatures are
encountered. This is fairly easy in film cameras, or in plate cameras
like the deRam, where the entire load of plates is carried in the camera
body. Plates carried in magazines present a more difficult problem. The
heating coil incorporated in the German cameras is perhaps partly for
this purpose.

=Color Sensitiveness.=—Complete specifications for an aerial plate
cannot be made solely on the basis of its speed, contrast, latitude,
threshold, and other sensitometric values which have to do only with the
intensity of the light acting on it. These in general apply to
photography from low altitudes, where the illumination and natural
contrast of the subject are the only factors to consider. When higher
altitudes are reached the interposition of haze decreases the already
deficient contrast, calling either for the development of more contrast
in the plate, or for the use of color filters to cut out the action of
the blue and violet light predominant in haze. Along the lines discussed
in the last section, it is not surprising to find that some plates are
better than others for bringing out gradations masked by haze, even
though no filters are used and though the plates are similar in color
sensitiveness. But the limitations to securing contrast by manipulating
the characteristic curve of the plate are soon reached, and it becomes
necessary to resort to haze-piercing color filters, used with _color
sensitive_ plates.

Roughly, two general types of color sensitive emulsions may be
distinguished: first, those in which sensitiveness to green and yellow
is added to the natural blue sensitiveness, and second, those sensitive
in a useful degree to all colors of the spectrum. The former are called
_iso-_ or _ortho-chromatic_, the latter _panchromatic_ emulsions.
Spectrograms exhibiting the distribution of sensitiveness throughout the
spectrum for several representative plates are shown in Fig. 105.
Orthochromatic plates are adequate for use with light yellow filters and
have the slight practical working advantage that they can be handled by
red light. Panchromatic plates are necessary for use with dark orange or
red filters. They must be handled in total darkness or in an exceedingly
faint blue-green light, taking advantage of the common drop in
sensibility in that region of the spectrum. Plates can, indeed, be
sensitized for the red alone, leaving a gap of almost complete
insensibility in the green, as shown in the fourth spectrogram of Fig.
105. When used with a yellow filter these plates behave as do
panchromatic plates with a red filter.

[Illustration:

  FIG. 105.—Spectrograms of representative photographic plates: _a_,
    ordinary plate; _b_, orthochromatic plate; _c_, specially
    green-sensitive plate; _d_, red sensitive plate, insensitive to
    green; _e_, panchromatic plate; _f_, specially red-sensitive
    panchromatic plate.]

A rougher idea of color sensitiveness than is given by spectrograms is
furnished by the _tri-color ratio_, which is the ratio of exposure times
necessary with white light to give equal photographic action through a
certain set of red, green and blue filters, expressed in terms of the
blue exposure as unity. In an excellent panchromatic plate the three
exposures would be equal. In an orthochromatic plate the red exposure
will be too large to be figured. In interpreting either spectrograms or
tri-color ratios care must be taken that the _absolute_ exposures
necessary are known. Thus a relatively high red sensitiveness may mean
merely low absolute blue sensitiveness.

Two methods are used in imparting color sensitiveness. Either the
sensitizing dye is incorporated in the plate emulsion before it is
flowed; or the plate is bathed in a dye solution not long before using.
The latter method gives higher color sensitiveness but poorer keeping
quality, and is not a practical method for field operations. Greatly
enhanced sensibility may be given by treatment with ammonia, but this
again is a method for laboratory rather than field use.

=Resolving Power.=—A question which arises in connection with all
photography of detail is the size of the grain of the photographic
emulsion. Dependent on the size of the grain is the _resolving power_,
or ability to separate images of closely adjacent objects. This varies
with the speed, fast plates being of coarser grain than slow ones; with
the exposure; and with the method and time of development. In general,
it may be said that the resolving power of the plate does not enter
practically into aerial work, because the resolving power of all plates
so far found usable corresponds to a smaller distance than the size of a
point image as limited by the performance of the camera lens and the
speed of the plane. Remembering that ⅒ mm. is a fair value for the size
of a point image as rendered by the lens, the rôle of plate-resolving
power is shown by consideration of the following table. Resolving powers
are given in terms of lines to the millimeter just separable.

                       Emulsion.        Resolving Power.
                Seed Graflex                   25
                Eastman Aerial Film            37
                Hammer Ortho                   44
                Cramer Isonon                  48
                Cramer Spectrum Process        57
                Eastman Portrait Film          61

=Tabulation of Requirements for Aerial Emulsions.=—In terms of the
sensitometric quantities just discussed the general requirements for
aerial plates may be listed as follows:

1. _Speed._ The speed usually connected with the contrast and density
required for the exposure times available is about 150 H & D. Faster
plates in general have too low contrast, but the highest speed that will
give the necessary contrast is desired.

2. _Contrast._ The contrast capable of development without fog should be
from 1.5 to 2. This contrast should be produced by light of daylight
quality, and, in orthochromatic and panchromatic plates, with the yellow
or orange filters intended to be used with them. This contrast means a
gamma infinity approaching 2.5.

3. _Speed of development._ A gamma of nearly 2 should be developed in 2½
minutes at 20 degrees C. in the developers recommended below.

4. _Fog._ Not over .25 for this degree of development, and not over .40
for six minutes development.

5. _Color sensitiveness._ This should in general be as high as possible.
In terms of certain representative filters (described in a subsequent
chapter) color sensitiveness should be such that with the white light
speed above specified the relative exposures through the filters shall
not be greater than as follows:

                           No filter Aero 1 Aero 2 #21 #23a #25
        Panchromatic plate     1      3      4.5    7   9   12
        Ortho plate            1      2.5    3.5    6

=Relative Behavior of Plates and Films.=—The advantages of film from the
standpoint of weight and bulk have been discussed in connection with
aerial cameras. Were there no other considerations film would
unquestionably be the most appropriate medium for aerial photography.
There is, however, the question of ease of handling, to be treated in a
subsequent chapter, and the question whether the purely photographic
characteristics of film are satisfactory. Can the same speed, contrast,
and color sensitiveness be obtained on film as on glass? Is the picture
so obtained as permanent or reliable as the plate image?

It must be confessed that up to the present emulsions on film have not
proved the equal of those on glass. It has been found by emulsion
manufacturers that the same emulsion flowed on film and on glass gives
better quality on the glass. Emulsions specially prepared for film fall
somewhat short of the best plate emulsions. It has also been found
harder to color-sensitize film, and to insure good keeping quality in
the color sensitized product.

In addition to the question of photographic quality there arises the
matter of shrinkage and distortion. These are negligible with plates,
but are a more or less unknown quantity in film. Irregular shrinkages of
as much as two per cent. are found on experiment. This defect, of
course, would be an obstacle only in exact mapping work.

=Positype Paper.=—The need sometimes arises in military operations to
secure prints ready for examination within a few minutes after the
receipt of the negatives. Even the 15 or 20 minutes within which a
negative can be developed and a wet print taken may be considered too
long. While such occasions are probably more apt to occur in popular
magazine stories than in actual warfare, it is important to have
available methods of producing prints with an absolute minimum of delay.
This need is met to some degree by a direct print process, commercially
exploited under the name of “Positype.”

In this process the exposure is made directly on a sensitized paper or
card, which is developed, the image dissolved out, the residue exposed,
and again developed; thus furnishing a positive picture (reversed right
and left). The time necessary to develop a print ready for examination
need not be more than three minutes. Only a single print is available,
but this is all that would be called for under the extreme conditions
suggested. If later, copies are desired they may be made by the same
process.

=Plates and Films Found Satisfactory for Aerial Work.=—The following
plates and films have been found particularly good for aerial
photography. The list is not intended to be complete. Furthermore, it
may be expected to be soon superseded, as the efforts of various
manufacturers are directed toward developing special aerial photographic
plates.

Among orthochromatic plates: The Cramer Commercial Isonon, the Jougla
Ortho.

Among panchromatic plates: The Ilford Special Panchromatic, the Cramer
Spectrum Process.

Film: Ansco Speedex, Eastman Aero.




                              CHAPTER XIX
                                FILTERS


=The Function of Filters in Aerial Photography.=—The use of color
screens or filters has been very common in ordinary landscape
photography, for the purpose of securing approximately correct
renderings of the brightnesses of colored objects. Plates of the
non-color-sensitive type have their maximum of sensitiveness in the blue
of the spectrum (Fig. 105) and in consequence blue skies photograph as
white, while other colors are likewise reproduced on a totally wrong
scale. Filters for correct brightness rendering are calculated for a
given color sensitive plate so that the resultant reaction to the light
of the spectrum copies the sensitiveness of the eye, which is greatest
in the yellow-green. Such filters for use with the common orthochromatic
plates are of a general yellow color.

Filters for aerial work are meant to serve quite a different purpose.
Correct tone or color rendering is of quite secondary importance to
another use of filters, namely, to cut or pierce aerial haze. It is
quite a matter of accident that the same general color of filter is
called for both to give correct color rendering and to pierce aerial
haze, namely, _yellow_. Yet on closer analysis it is found that quite
different types of yellow filter are demanded, spectroscopically
considered.

Figure 106 (K_{1} and K_{2}) shows the spectral transmission curves of
the Wratten K_{1} and K_{2} filters, intended for correct color
rendering with orthochromatic plates. The absorption increases
_gradually_ toward the blue. In the same figure is shown on an arbitrary
scale the spectroscopic character of typical haze illumination,
increasing in brightness inversely as the fourth power of the
wave-length, that is, with great rapidity in the blue and violet. It is
evident from this that a much more abrupt absorption than that of the
K_{1} or K_{2} filter is desirable, because in the green of the spectrum
the haze light is comparatively weak, and more will be lost by any
absorption in this region through decreasing useful photographic action
than will be gained by cutting out the haze. This latter consideration
is important. The use of any filter means an increase of exposure; the
use of yellow filters multiplies it several times. Careful experiment
has shown that no filter of depth less than K 1½, to use the Wratten
filters as a basis for discussion, are of real value in haze piercing.
The _filter ratio_, or ratio of exposures with and without filter, is
4.7 for the K 1½ with the Cramer Isonon plate—a figure which shows the
importance of securing the necessary haze-piercing character with the
minimum absorption of useful photographic light.

[Illustration:

  FIG. 106.—Characteristics of various filters.]

=Practical Filters.=—Since the character of the absorption of the “K”
filters is not all that could be desired, new filters, both of dyed
gelatin and of glass, have been produced. The glass, a Corning product
having a very sharp-cut absorption, has not yet been produced on a
commercial scale with the high transparency in green, yellow and red
that selected samples have shown. The United States Air Service has
adopted filters of a new dye, called the EK, from the name of the
company in whose laboratory it was produced. These filters are
standardized in two depths of staining, called the “Aero No. 1” and
“Aero No. 2.” Their spectral transmission curves appear in Fig. 106,
along with those of certain darker filters useful only with panchromatic
plates for exceptionally heavy haze. The characteristic of these Aero
filters is their great transparency through all the spectrum except the
blue, whereby the greatest haze-cutting action is attained together with
a low filter factor. The filter factors of the Aero No. 1 and No. 2 with
Cramer Isonon plates are 3 and 5, respectively.

=Effects Secured by the Use of Filters.=—The efficiency of yellow
filters for haze-cutting is best shown by photographs taken at high
altitudes with filters and without. Such illustrations are given in
Figs. 107 and 108, where the first photograph is one taken at 10,000
feet without a filter, the second taken at the same altitude under the
same conditions, but with an orange filter. Both are on panchromatic
plates, and it will be seen that even with these plates the filter makes
all the difference between a useless and a useful picture. But it must
be clearly understood that the difference here lies between a plate
sensitive chiefly in the blue and violet, and a plate affected only by
the yellow, orange and red. The difference is not between what the eye
sees and what a plate with a filter sees, as is sometimes supposed. As
shown in Fig. 108, a filter enables the plate to photograph through the
haze between clouds, but not through the clouds themselves. In general,
no filter and plate combination which is feasible for aerial exposures
is capable of showing more than the eye can see if yellow or orange
goggles are worn. To do this it would be necessary for the photographic
action to take place by deep red or infra-red light, which would demand
exposures now out of the question.

[Illustration:

  FIG. 107.—A photograph taken at 10,000 feet, without a filter.]

Filters are almost always necessary in photographing from high altitudes
or in making distant obliques. At times, particularly after a heavy
rain, the air is clear enough so that filters may be dispensed with.
Clearing weather was therefore chosen whenever possible for making
obliques of the battle front.

[Illustration:

  FIG. 108.—Photograph taken at same time and over same neighborhood as
    Fig. 107, but with an orange filter.]

=Filters for the Photographic Detection of Camouflage.=—In the
photographic as in the visual detection of camouflage, the problem is to
differentiate colors which ordinarily look alike, but which are actually
of different color composition. Particularly important are the
differences between natural foliage greens and the paints used to
simulate them. If these differ in their reflection spectra, a proper
choice of filter will show up the two greens as markedly different. Two
kinds of difference may be produced; either the two colors may be
changed in relative brightness, or they may be altered in hue. Thus
foliage green, due to its possessing a reflection band in the red of the
spectrum, which is absent in most pigments, may be made to appear _red_
while the camouflage remains green or turns black. Filters which cause
changes of color are of course of no use for photographic detection of
camouflage, since the photographic image is colorless. Brightness
differences are alone available.

Those same filters which have been worked out primarily for producing
brightness differences in visual detection of camouflage could be used
photographically, provided the plates employed were color sensitive, and
were as well screened to imitate the sensibility of the eye. But the
most useful visual filters are those causing color differences to
appear; more than this, the visual camouflage detection filters as a
class have low light transmissions, so that their usefulness in
photography is doubtful. Little work has actually been done with
camouflage detection filters for photography. Yet in spite of this
photography has been of real service in this form of detective work. Its
utility for the purpose comes from the fact that the natural
sensitiveness of the plate to blue, violet and invisible ultra-violet
acts to extend the range of the spectrum in which differences between
identical and merely visually matched colors may be picked up.
Consequently the plain unscreened plate has proved a very efficient
camouflage detector—so efficient in fact that all camouflage materials
have had to be subjected to a photographic test before acceptance. Fig.
171 shows how an ordinary photograph reveals the unnatural character of
the camouflage over a battery.

=Methods of Mounting and Using Filters.=—The most primitive way of
mounting a gelatin filter is to cut a disc from a sheet of dyed gelatin
and insert it between the components of the lens. For this purpose the
gelatin must be perfectly flat, which is insured by its method of
preparation and test. One disadvantage of this method is that the filter
can be inserted and removed only upon the ground. It is less
satisfactory the larger the diameter of the lens, and the wastage of
filters due to insertion and removal is apt to be high. The camera
should be refocussed after filters of this kind are inserted.

Glass filters, ground optically true, or gelatin filters, mounted
between optically flat glass plates, are the most convenient and
satisfactory. They may be mounted in circular cells to screw or attach
by bayonet catches to the front of the lens. Or they may be mounted in
rectangular frames to slide into transverse grooves in the camera body.
Fig. 44 shows the mount of this latter form adopted in the larger United
States Air Service cameras. This is particularly convenient if it is
desired to insert or change the filter while in the air—a practice not
generally considered feasible in war work with the photographically
inexperienced observer, but likely to be common with the employment of
skilled photographers for peace-time aerial photography.

German cameras are reported in which the glass filter is carried behind
the lens, on a lever which also carries a clear glass plate of the same
thickness, to be thrown in when no filter is needed, thus maintaining
the focus. The performance of the lens will be impaired by this scheme,
unless it is specially calculated to offset the effect of the glass
introduced in the path of the rays behind the lens—optically true glass
has no effect on definition if placed in front of the lens. Glass
filters may also be placed in close contact with the plate or film, in
which case they must be much larger, but do not need to be of as good
optical quality.

=Self-screening Plates.=—Mention must be made of a quite different mode
of realizing the filter idea, a method available where the sensitive
plate is always to be used with a filter. This is to incorporate a
yellow dye in the gelatin of the plate itself. The dye must be one which
has no direct chemical effect on the plate, but which acts simply as a
coloring agent for the gelatin. “Self-screening” plates, as they are
called, have been produced by the use of the dye called “filter yellow”
and have found some use in orthochromatic photography. They effect a
useful saving of light through the elimination of the reflection losses
at the surfaces of glass and gelatin filters. The filtering action of
the dye in the plate is somewhat different from its ordinary one, since
the deeper portions of the sensitive film are subject to greater action
than the surface, and this tends to diminish contrast.




                               CHAPTER XX
                      EXPOSURE OF AERIAL NEGATIVES


The principal factors governing the length of exposure in the airplane
camera have already been discussed under various headings. These are
briefly, the nature of the aerial landscape, the practically attainable
lens apertures, the form of the camera support, the speed of the plane,
and the characteristics of plates, films and filters. It is convenient
however, to re-assemble this information in one place, in such form as
to apply to the practical problem of determining the exposure to be
given in any specific case.

=Limitations to Exposure.=—In the ordinary photography of stationary
objects, exposure is a variable entirely at the operator's command.
Plates of any speed may be selected, so that attention may be focussed
on latitude, color sensitiveness, and other tone rendering
characteristics. The exposure may be made of a length sufficient to
insure all the useful photographic action lying in the “correct
exposure” portion of the sensitometric curve. The exposure ratio of any
filter it is desired to use is a matter of indifference—its effect on
color rendering need alone be considered.

Airplane photography is sharply distinguished from ground “still”
photography by its severe limitations as to the amount of the exposure.
The actual duration is definitely restricted by the high speed of the
plane. In peace work this can be offset in part by using slower planes
or by flying against the wind. The practical limitation to 1/100 second,
set by war-time requirements as to definition of fine detail, may be
increased to 1/50 of a second, or even more, where mapping of grosser
features is the object. A common, but entirely avoidable limitation, is
that due to vibration of the camera. By proper mounting this may be
entirely overcome, leaving the ground speed of the plane the only source
of exposure-limiting movement. The amount of light reaching the plate
constitutes a primary factor in exposure, and this is a matter of lens
aperture. Generally, lens aperture is smaller the larger the plate
required to be covered, and the greater the focal length. Because of
their larger aperture, the short-focus lenses which will be favored for
peace-time large-area mapping will permit more and longer working days
than have been the rule in long-focus war photography. The necessary use
of filters, particularly at the high altitudes which would be chosen in
mapping, in order to economize in the number of flights needed to cover
a given area, introduces an inevitable decrease in the amount of light
available at the plate, as compared with surface photography under the
same illuminations.

Broadly speaking, it may be said that all the demands made in reference
to aerial photographic exposure work are to _decrease_ the amount of
light reaching the plate. Any surplus offered, as by the midsummer
noon-day sun, must be immediately snapped up, either by decreasing the
exposure to get greater sharpness, or by introducing filters to get
greater photographic contrast. The absolute exposure of the plate tends
to be kept at the irreducible minimum. As already stated, it lies, with
present photographic materials, on the “toe” of the “H & D” curve, just
reaching up into the straight line portion.

=Estimation of Exposure.=—According to the foregoing argument the
problem of estimating an aerial exposure resolves itself largely into
one of deciding how short this may be made. Or, if the light is strong,
whether it is sufficient so that a filter may be introduced without
demanding more than the 1/100 second or thereabouts which is dictated by
the motion of the plane.

Deciding upon exposures in the field has been largely a matter of
experience and judgment. A majority of the cameras in use during the war
were not furnished with shutters calibrated in definite speeds.
Consequently, the sergeant upon whom the decision usually devolved
became a storehouse of knowledge as to the slit widths and tensions
appropriate to each individual camera. This knowledge had to be acquired
from the results of actual photographic reconnaissances, or from special
test flights, both of them wasteful methods. But the chief objection to
this state of affairs lies in the fact that the knowledge thus acquired
is of no use to anyone else, nor is it applicable to other types of
camera.

The first essential to placing exposure estimation upon a sound basis is
therefore an accurate knowledge of shutter performances. Either the
shutter speeds should be placed upon the camera by the manufacturer and
periodically checked, or a regular practice should be followed of
calibrating shutters, either at a base laboratory or even in the field.

Assuming that the speeds of all shutters are accurately known, the
process of estimating the requisite exposure becomes less a matter of
mere guesswork and more nearly a matter of precision. For this purpose
data on the variation of light intensity during the day and during the
year (Figs. 101 and 102) should be taken as a guide. These data refer of
course to visual and not to photographic light, but since it is always
necessary to use color filters, which make the active light of
approximately visual quality, this is no valid objection. The effects of
clouds and mist must of course be learned largely by experience, but
with the above daylight data at hand, anyone in possession of definite
information on the correct exposure with a given plate for a known day
and hour need not go far wrong in estimating exposures at any other time
in definite fractions of a second.

[Illustration:

  FIG. 109.—Chart showing aerial exposures for all times of the day and
    year. Data on basis of F/5.6 lens, Jougla orthochromatic plate, and
    clear sunlight, no filter. Exposures to be doubled and tripled for
    overcast and cloudy weather.]

_Exposure data charts._ Fig. 109 shows a chart, prepared in the French
service, indicating aerial exposures for all hours of the day throughout
the year. These are for clear sunlight, for a lens of aperture F/5.6 and
for “ortho” plates without a filter. They are based on what is probably
an over-estimate of the actual speeds given by the French shutters. For
“light” clouds the exposures are to be doubled, for “heavy” clouds
quadrupled, and for forests and dark ground “lengthened.” Charts of this
form should be extremely useful, but they were actually not of great
service because of the prevalent lack of knowledge of true shutter
speeds.

_Exposure meters._ Aerial photography offers an excellent opportunity
for the use of exposure meters, particularly those of the type in which
a sensitive surface is exposed to the light for a measured time
sufficient to darken a predetermined amount. The sensitive paper of the
meter may either be exposed from the ground to the direct light of sun
and sky, or from the plane to the light reflected from the ground. The
first method will give figures subject to some correction for the
character of the ground to be photographed—whether fields, forests, or
snow. The second method is to be preferred where the shutter speed can
be adjusted in the air, according to the indications of the meter, or
where the filter can be selected and put in place during flight. Trials
with a commercial Wynne exposure meter, used in the latter manner, give
as a working figure an exposure of .001 second for each 4½ seconds taken
to darken the sensitometer strip to match the darker comparison patch.
This relation applies to a lens of aperture F/4.5, on Cramer Commercial
Isonon plates without filter.




                              CHAPTER XXI
                             PRINTING MEDIA


Skilled photographers can examine a negative and can interpret its
renderings with practically as much satisfaction as they get from a
print, whereby a considerable amount of time can be saved in an
emergency. The original glass negative should always be used when
accurate measurements are to be made. These and a few other cases
constitute the only use of a negative apart from its normal one, namely,
for producing positive prints, usually in large numbers. The commonest
form of print is on paper, although the most satisfactory print from the
photographic standpoint is the transparency on glass or celluloid film.

=Transparencies.=—Transparencies are made by the regular photographic
processes of exposure and development, on glass plates or films placed
in contact with the negative, or in the appropriate position in an
enlarging camera. The sensitometry and the terms used to describe the
qualities of plate or film for this purpose are those already given in
connection with the general discussion of plates and films. But the kind
of emulsion to be selected is quite different from the aerial negative
emulsion. There is here no practical limitation to the speed, contrast
or latitude. Consequently, we can choose a positive emulsion on which
the exposure through the aerial negative falls entirely on the straight
line portion of the characteristic curve, thus reproducing all of its
tones, and the contrast of the negative may be increased to any desired
extent. The possibilities of positive emulsion are indeed rather greater
than the usual aerial negative can utilize. A range of clearly graduated
opacities of two or three hundred to one is possible, so that not only
can detail be well rendered in the high-lights, but also equally well in
dark shadows where, indeed, an increase of illumination is necessary for
it to be made easy to examine. This range is to be contrasted with the
1-to-7 range in the aerial landscape, which may be doubled by a
contrasty plate. In resolving power, the positive emulsion, which is
slow, exceeds the negative emulsion. It easily bears examination through
a magnifying glass, thus making any enlargement unnecessary in the
printing process.

Glass transparencies are of course impractical for general distribution,
on account of their fragility. Heavy film transparencies are not open to
this objection, and, especially in the form of stereos, constitute the
most beautiful form of aerial photographic print.

=Paper Prints.=—Prints on paper suffer by comparison with
transparencies, in the range of tones which they exhibit. This lies
between the white of the paper, which never has more than 80 per cent.
reflecting power, and its darkest black, which differs with the kind of
paper. In dull or mat papers the blacks will reflect as much as 5 per
cent.; in glossy papers, ordinarily used for aerial negatives, the
reflection from the black may be as low as one per cent., but in order
to get the benefit of this the paper must be so held as not to reflect
any bright object to the eyes. This deficiency in the range of paper
gradations is not so serious with aerial negatives as with ordinary
properly exposed negatives because of the small range of brightness in
the aerial view.

The sensitometry of papers is similar to that of plates, with the
difference that reflecting powers take the place of transparency. As in
the case of transparency emulsions there is in papers no dominating
requirement for extreme speed, to which other characteristics must be
subordinated. Yet speed is of sufficient importance in handling large
quantities of prints so that aerial negative printing for military
purposes has been done almost entirely on the rapid enlarging papers,
rather than on the true contact printing papers, which are slower.

[Illustration:

  FIG. 110.—Characteristic curves of bromide paper.]

The two principal types of rapid enlarging papers, the bromide and the
“gas light,” exhibit certain characteristic differences which are
important to bear in mind in seeking to obtain any particular quality of
print. Bromide papers, of which “Nikko” is a good example, show
sensitometric curves rather like those of plates. That is, they increase
in contrast with continued development. At the same time, as is shown in
Fig. 110, they increase somewhat in speed with development; that is,
under exposure can be compensated for to a small degree by protracted
development. These characteristics of bromide paper can be utilized to
secure prints of a quality quite different from that of the negative.
Thus, if the negative has a long range of tones, a flat print can be
secured by full exposure and short development. If, as is apt to be the
case with aerial negatives, a print of greater contrast than the
negative is desired, a short exposure with long development is called
for.

[Illustration:

  FIG. 111.—Characteristic curves of gas light paper.]

The sensitometric curves of a typical gas light paper “Contrast
Enlarging Cyco,” are shown in Fig. 111. Here the contrast is a fixed
characteristic of the paper, and the only effect of changing development
is on the speed; that is, exposure and development are, within limits,
interchangeable.

Choosing a printing paper is a matter of deciding on the contrast
required for the class of negative, and selecting a paper which will
give this contrast with a good range of tones from a clear white to a
deep black. The ideal paper would be one which was all straight line in
the H & D plot. In such a paper there would occur no loss of contrast in
the lighter tones when the high-lights were rendered by the clear white
of the paper. Too great contrast with a short straight line portion,
results in loss of detail at the ends of the scale. A negative
possessing a very great range of tones cannot be correctly represented
on one paper print—two printings are required, one for high-lights and
one for shadows, but this difficulty is rarely to be faced in aerial
views. The greatest demand for aerial printing papers has been for those
of considerable contrast, because of the flat character of the
negatives.




                              CHAPTER XXII
                         PHOTOGRAPHIC CHEMICALS


=General Considerations.=—Developing, fixing and other chemicals for
aerial work differ in no essential respect from those used in ordinary
photography. Full discussions of these are to be found in numerous texts
and articles. The aerial photographic problem is to select those most
suited for the under-exposed flat negatives characteristic of
photographs from the air. At the same time selection from among the
chemicals of appropriate quality must be governed by considerations of
the conditions surrounding work in aerial photographic laboratories.
These laboratories, especially in war-time, are apt to be most primitive
in their facilities.

=Characteristics of Developers for Plates and Films.=—From the
standpoint of practicability, aerial negative developers should have
good keeping power, be slow to exhaust, and work well over a
considerable range of temperatures. From the standpoint of the
photographic quality desired in the negative, the developer should bring
up the maximum amount of under-exposed detail. This means that it should
impart the highest possible speed to the plate, with good contrast, and
low fog or general reduction of unexposed silver bromide.

There are many characteristics to study in a developer: its effect on
inertia or speed, gamma infinity, fog, time of appearance, “Watkins
factor,” speed of development, temperature coefficient, dilution
coefficient, keeping power, exhaustion, length of rinsing, stain, color
coefficient and resolving power. These are defined and described as
follows:

_Effect on inertia._ The meaning of inertia has already been given under
the discussion of plate speed. While this is a constant, independent of
time of development, for any one developer, it is altered appreciably by
change of the latter.

_Time-gamma relation._ Contrast, symbolized by γ, has likewise been
discussed under plate sensitometry. Viewed from the standpoint of the
developer, the point of interest is the rate at which γ varies with
development, and the maximum contrast which can be reached or γ
infinity. Speed of development is commonly defined by the _velocity
constant_, symbolized by κ, which is arrived at mathematically from a
consideration of the time of development to produce two different
contrast values. High γ infinity is desired for aerial negatives, and
for rapid work κ must also be high.

_Fog._ The opacity due to chemical fog is to be kept at a minimum in
aerial negatives, as it is chiefly prejudicial to under exposures.

_Time of appearance and Watkins factor._ The time of appearance is
measured in seconds. The Watkins factor is a practical measure of the
speed of development, and is determined by the ratio of the time of
development required for a definite contrast, to the time of appearance.
It is useful also as a guide to development time.

_Temperature coefficient._ This is the factor by which the time of
development at normal temperature (20 Cent.) must be increased or
decreased in order to obtain the same quality negative, for a change of
seven degrees either side of normal.

_Temperature limits_ are the temperatures between which development can
be carried out with any degree of control or without serious damage to
the negative. These factors are of great importance where climatic or
seasonal changes have to be endured.

_Dilution coefficient._ This is the factor by which the development time
is increased in order to maintain a given quality negative in different
dilutions of the developer. It is useful in tank development.

_Keeping power._ The keeping power of a developer, mixed ready for use,
is determined by its ability to resist aerial oxidation. A developer of
poor keeping power, which must be made up immediately before use, causes
delay and waste of time whenever emergency work has to be done, whereas
a developer of good keeping power may be left in its tank ready for
instant use.

_Exhaustion_ of a developer is the rate at which it becomes useless for
developing, due both to aerial oxidation and to the using up of its
reducing power by the work done in developing plates. It is conveniently
measured by the area of plate surface developable before the solution
must be renewed.

_Length of rinsing._ The time required for rinsing between development
and fixing bath plays a not unimportant part in total development time.
Dichroic fog is caused with some developers if, due to insufficient
rinsing, any of the caustic alkali is carried over to the fixing bath.
Stains develop also if the fixing bath is old, or if light falls on the
unfixed plate while any developer remains in the film.

_Color coefficient._ The function of the sulphite, which forms a
constituent of all developing solutions, is two-fold. It acts partly as
a preservative, and partly to prevent the occurrence of a yellow color
in the deposit. The yellow color, if present, increases the photographic
contrast. This phenomenon has been purposely utilized, particularly in
the British service, to give “stain” to negatives which otherwise would
show insufficient printing density. The color index or coefficient of a
negative (with a given printing medium) is the ratio of photographic to
visual density. If we take a pyro developer containing five parts of
pyro per thousand and ten parts of sodium carbonate, and then vary the
amount of sulphite from none to fifty parts per thousand, the color
index varies as follows:

                          Sulphite            Color Index
                     Parts per Thousand
                             50                  1.16
                             25                  1.24
                             15                  1.30
                             10                  1.45
                              5                  1.80
                              0                  2.75

The color index is somewhat different with various kinds of printing
media.

This staining effect is a variable one, depending upon length of
development, dilution of the developer, length of rinsing, temperature,
the fixing bath used (plain hypo being necessary for a maximum effect),
the length of washing after fixation and the properties of the water
used. Standardization of these conditions in the field is difficult;
hence any developer which will give the same effective contrast without
resorting to stain is to be preferred.

_Resolving power._ Some developing processes and conditions will
introduce bad grain into the negative. Hence the resolving power which a
developer brings up must be investigated among its other
characteristics.

=Practical Developers for Aerial Negatives.=—In the English service a
pyro metol developer was generally used, producing stained negatives.
The French, American and Italian practice was to use metol-hydrochinon,
without staining. A special chlor-hydrochinon developer, worked out by
the Eastman Research Laboratory for the United States Air Service, has
probably the greatest merit of any yet tried. A comparison, given below,
between it and a pyro metol formula used on a representative plate,
illustrates the use of the various bases of study given above.

                              PYRO FORMULA

                   Solution A                  Solution B
        Pyro, 3.75 grams                 Sodium carbonate, 53 g
        Potassium metabisulphite, 3.75 g
        Metol, 3.05 g
        Potassium bromide, 1.5 g
        Water, 500 c.c.                  Water, 500 c.c.

                       Use 1 part of A to 1 of B

                        CHLORHYDROCHINON FORMULA

                   Solution A              Solution B
            Chlorhydrochinon, 25 g   Sodium carbonate, 30 g
            Metol, 6 g               Sodium hydrate, 10 g
            Sodium bisulphite, 2.5 g Potassium bromide, 3 g
            Sodium sulphite, 25 g
            Water to 670 c.c.        Water to 330 c.c.

                       Use 2 parts of A to 1 of B

                                     Pyro     Chlorhydrochinon
          H & D speed             150         180
          Gamma infinity          1.45        2.12
          Fog (at maximum gamma)  .32         .60
          Time of appearance      5 seconds   5 seconds
          Watkins factor          25          10
          Velocity factor “κ”     .320        .400
          Temperature coefficient 1.40        2.0
          Temperature limits      4° to 32° C 4° to 32° C
          Keeping power           45 minutes  8 days
          Exhaustion (100 c.c.)   30 sq. in.  300 sq. inches
          Dilution coefficient    2           2
          Color coefficient       1.50        1.00
          Resolving power         47          53

Owing to the difficulty of securing pure chlor-hydrochinon a metol
hydrochinon of very similar properties has been worked out. Its
composition is

                       Metol             16 grams
                       Hydrochinon       16 grams
                       Sodium sulphite   60 grams
                       Sodium hydroxide  10 grams
                       Potassium bromide 10 grams
                       Water to           1 litre

To keep the ingredients in solution in cold weather, 50 c.c. of alcohol
should be included in every litre of solution. All things considered
this is probably the most practical and satisfactory developer for
aerial negatives.

=Developers for Papers.=—The following formula has been found very
satisfactory for papers:

                   Metol                    .9 gram
                   Hydrochinon             3.6 gram
                   Sodium carbonate       20.0 gram
                   Sodium sulphite        14.0 gram
                   Potassium bromide .5 to 1.0 gram
                   Water to                1   litre

=Fixing Baths.=—For plates the following fixing and hardening bath is
recommended:

                 Sodium thiosulphate (hypo)  350 grams
                 Potassium chrome alum         6 grams
                 Sodium bisulphite            10 grams
                 Water to                   1000 c.c.

During hot weather, the above quantities of chrome alum and bisulphite
are doubled.

For papers the following:

                     Hypo, 35 per cent. 100 volumes
                     Acid hardener        5 volumes

The acid hardener is constituted as follows:

                       Alum             50 grams
                       Acid acetic 28° 400 c.c.
                       Sodium sulphite 100 grams
                       Water to          1 litre

=Intensification and Reduction.=—These processes have been little
employed in air work. Reduction is rarely necessary, for obvious
reasons. Intensification would often be of value, but the common
practice, which saves some time, is to use printing paper of strong
contrast for those negatives which are deficient in density and
contrast. When intensification is desirable or permissible, either the
ordinary mercury or uranium intensifier may be used.

=Water.=—In the field it is found necessary in many cases to purify the
water that is to be used in mixing up chemicals. Water may contain
suspended matter or dirt, dissolved salts, and slime. It is important to
remove the suspended matter, as it may cause spots on the plates and
papers, while any slime would coagulate, forming a sludge in the
developer which would also tend to settle on the plates and cause marks
during development. The dissolved salts may or may not cause trouble.
Two methods of purification are possible:

(_a_) Filter the water through a cloth into a barrel, add about one gram
of alum for every four litres of water, and allow to settle over night.
Draw off the clear liquid from a plug in the side as required.

(_b_) Boil the water and allow it to cool over night. If the water
contains dissolved lime, boiling will often cause this to come out of
solution.




                                   V
              METHODS OF HANDLING PLATES, FILMS AND PAPERS




                             CHAPTER XXIII
             THE DEVELOPING AND DRYING OF PLATES AND FILMS


=Field Requirements.=—Developing, fixing, drying and printing in the
field demand simple and convenient apparatus that may be carried about
and installed with the least amount of labor. On top of these
requirements military needs impose others that are more difficult.
_Speed_ is, on occasion, imperative. A print may be required within a
few minutes after landing, and many thousands within a few hours.
Quantity production must be achieved under the most primitive
conditions. Nothing, in fact, shows the calibre of the photographic
officer better than his choice of workplaces as the army moves forward.
Ingenuity and practical judgment are at a premium. Cellars, stables, dog
kennels, or huts hastily built from packing cases, must be equipped and
in working order over night. All the facilities offered by a great city
are urgently needed—water, electric light, power for driving fans—but
must be dispensed with if the photographic section is to be convenient
to the airdrome, whose portable hangars are most apt to be pitched in
the open country. Water must be carried, electricity generated, and to
the photographic problem is added the military one of concealment and
protection. Dugouts and bomb proofs must be built for supplies, and
“funk holes” for the men. Entire underground emergency extensions have
sometimes been built in stations occupied for extended periods, for
airdromes are a favorite bombing target.

For getting the exposed plates to the photo section, messengers, on
motorcycles if possible, are employed. In some cases, where hangars and
photographic hut are forced to be widely separated, recourse has been
had to parachutes (Fig. 112), a device also employed to distribute
prints to infantry during an advance.

[Illustration:

  FIG. 112.—Receiving pictures from plane by parachute.]

For warfare of movement, especially in sparsely settled or devastated
country, where cellars are unavailable, the dark room must be taken
along. _Motor trucks and trailers_ (Figs. 113, 114, 115), the former for
hauling supplies and electric light generating plant, the latter fitted
as a complete developing and printing laboratory, form the headquarters
of each photographic section in the field. Usually altogether too small
for the amount of work required, they were extended by tents and
lean-to's, or ingeniously used as a nucleus for the organization of the
favored stable or cellar.

[Illustration:

  FIG. 113.—Mobile photographic laboratory.]

=Methods of Plate Development.=—Where speed is not required the simplest
and commonest mode of developing plates is in the tray, one plate at a
time. Common practice is to examine the plate at intervals during
development, and discontinue the operation on the basis of its
appearance. This is only possible if the plates used are insensitive to
some light by which the eye can see. Deep red light is suitable for
ordinary and most orthochromatic plates. A faint blue-green may be used
with some panchromatic plates. The best practice, however, is to develop
by time in total darkness, whereby all chance of dark room fog is
avoided. Development time for plates of the average exposure of the one
to be developed is either known from previous experience, or is found by
trial on the first one. Development by time results in negatives of
densities varying with the exposures, but, as was brought out in the
discussion of sensitometry, this difference can be compensated for by
the choice of the paper used for printing, and by its treatment.

[Illustration:

  FIG. 114.—Interior of photographic trailer, developing room.]

Where larger quantities of plates are to be handled _tank development_
is adopted. In ordinary tank development the plates are placed in
grooved tanks, into which is poured first the developer, next the
rinsing water, and then the hypo. It has been customary in tank
development as practiced for peace-time work to use dilute developer,
requiring from ten to thirty minutes, but speed requirements in war-time
aerial photography dictate the use of full-strength quick-acting
developer. An improvement on the simple grooved tank is provided by
metal cages or racks, each holding a dozen or more plates, which may be
introduced or removed from the tank as a unit (Fig. 116).

[Illustration:

  FIG. 115.—Interior of photographic trailer. Enlarging camera and
    printer.]

The _core rack_ system combines certain of the features of both tray and
tank development. Each plate is inserted in a separate metal frame with
projecting lugs to rest on the top of the tank and so suspend the plate
in the solution. The process of development is the same as in the tank
system, but any individual plate may be examined and removed.

[Illustration:

  FIG. 116.—Tank and rack for tank development.]

=Film Developing and Fixing.=—The problem of quickly handling roll film
of large size is one upon whose solution depends in large degree the
feasibility of film cameras for aerial work. It presents many
difficulties: a long film is unwieldy, is inherently subject to curling,
and takes up much space if it is handled entire. For small scale
operations roll film can be cut into short strips and developed either
by drawing through a tray or, if cost of developer is no object, in a
deep tank. In order to make the cutting apart of exposures easy in the
dark, film cameras should make some form of punch mark in the film
between the exposed parts, or the space between exposures should be
uniform, so that a print trimmer set to a definite mark may be used.
Racks for holding two or three feet of film, folded back on itself and
clasped by spring clothes-pins, are fairly practical. One object of the
use of film, however, is to greatly increase the number of possible
exposures; and where hundreds instead of dozens of exposures are to be
developed, this method takes up entirely too much time.

Following the practice in moving picture development, _film developing
machines_ of various designs have been devised. Among these may be
described the G. E. M. machine; the Ansco machine; the Eastman apron
machine; the Brock frame and tank apparatus; the Eastman reel machine;
and a modification of the latter by the United States Air Service.

The G. E. M. film developing apparatus, similar in idea to the Eastman
“apron” method of film developing, as exemplified in the familiar
amateur film developing machines, has the film wound in a spiral on a
long linked metal frame or chain. After being wound it is placed in a
tub of developer, from that to a tub of water, thence to a tub of hypo,
and finally to a tub of water, where it is washed in several changes.
The objections to the method are that it takes up much floor space for
the various tubs, and that it requires such large quantities of
solution. To develop a thirty-five foot length of 18 × 24 centimeter
exposures requires approximately 28 gallons of developer; for the
rinsing, 28 gallons of water, and the same for hypo, and at least three
times that for washing. In all 168 gallons of water must be brought to
the developing hut or lorry.

The Ansco machine makes use of an idea frequently applied in the moving
picture industry. The film is carried spirally, upon two cross-arms
which bisect each other at right angles, and which contain vertical pins
around which the film is looped, beginning at the center and working
out. After it is wound it is placed in a tub of developer, as in the G.
E. M. machine. It has an advantage over this apparatus in that the shape
of the tubs or tanks is square instead of round. But it is equally
extravagant of space and water.

This same criticism may be made of the Eastman apron apparatus for film
developing. This is similar to the G. E. M. machine, but differs from it
in using a perforated celluloid apron to support the film during the
various operations, instead of a metal chain.

The Brock developing outfit consists of a rectangular wooden frame and a
three-compartment tank. The frame, which is approximately 3 by 4 feet in
size, is used as a support for the 4 inch wide film, which is wound
spirally around it, between guiding pins. A special support is provided,
on which the frame may be rotated as the film is fed off the camera
spool. The frame, with the film on it, is lowered successively into the
three narrow but deep compartments of the developing tank. The first
compartment holds developer, the next water, the next hypo. The amount
of developing solution required is rather large (96 gallons of water in
all for a strip of 100 4 × 5 inch exposures), but because of the small
surface exposed to the air, it keeps for a considerable period. The
chief demand for floor space with this apparatus is for feeding the film
on to the frame.

In the Eastman twin reel machine the film is wound on a wooden drum or
reel of large diameter, to form a helix. The drum is suspended so that
the bottom edge touches the developing solution, and, upon revolving the
drum, every portion of the helix of film is brought into contact with
the developer. By shaping the developing tank so that it closely
conforms to the shape of the reels, a high economy in quantity of
developing agent can be achieved. When developing action is finished,
the developer is emptied out, rinse water put in; hypo follows, and then
comes the final washing with water. With this apparatus the whole cycle
is completed, for the 35 feet length of film above considered, with
seven gallons of water.

The Air Service apparatus differs from the above only in the drying
method, which will be described below.

Heavy cut film, such as is marketed under the name of Portrait Film, has
not thus far been used in aerial work, except for printing
transparencies. It is conceivable, however, that film in the cut form
may be used in some future design of camera. This may be developed
expeditiously in a tray, six or eight films being handled at a time, in
a pile, pulling out the lower one frequently and placing it on top. The
core rack system is also available for film in this form, special racks
with clips to hold the film being necessary.

=Plate Drying.=—The drying of negatives on glass is a comparatively
simple matter, owing to the rigid nature of the emulsion support. A
large number of plates may be placed in a compact mass in the ordinary
plate racks of commerce with the wet sides accessible to a draft of air.
Two dozen plates separated from each other by a quarter of an inch and
left to dry spontaneously in a room of ordinary humidity and living
temperature will dry in two hours and a half. If the surface be wiped
with soft cheese-cloth or chamois, so as to absorb all the surface
moisture before the plates are placed on the rack, this time may be
appreciably reduced. By placing the plates in a forced draft of air,
from an electric fan, this time may be reduced to an hour.

Extra rapid drying of plates may be accomplished by placing them in a
bath of alcohol before putting them in the racks. The alcohol displaces
all the water in the film, and is itself very quickly dissipated into
the atmosphere when the plate is taken from the tray. The plate must be
left in the alcohol tray long enough for the substitution of the alcohol
for the water in the film to take place. Five minutes is long enough.
The alcohol before use must be as nearly free from water as possible.
The best way to make sure of this is to place in the bottle of alcohol
some lumps of calcium oxide, which will take up the water and form
calcium hydroxide, which settles at the bottom of the bottle.

Another method of quick plate drying takes advantage of the
extraordinary greediness of potassium carbonate for water. The wet
plates are placed in a saturated solution of potassium carbonate and
left for a minute. If a plate be now taken from the solution and its
surface wiped with a soft cloth, it will be found that the film has a
greasy, slippery feeling, but that it contains no water and can be
printed from at once. Plates so treated should be washed, however, at
some time in the succeeding four months, or the traces of potassium
carbonate left in the film cause deterioration.

=Film Drying.=—Unlike the drying of plates, drying of film negatives is
a very puzzling problem, and may be considered as the crux of the
successful use of film in aerial cameras.

Apron and similar machines have very poor drying efficiency if the film
is left in place, for not only the film but the apron or chain must be
freed of water. This may be hastened, as in the G. E. M. machine, by
blowing air through with fans, but even with their help drying a 35 foot
film is a matter of two hours or more. Passing the film through wringers
or a squeegee to remove excess water is a considerable aid; the film may
either be re-wound on a dry reel, to be put in a forced draft of air, or
may be hung up in short lengths or festooned, either method taking up a
great deal of space. The use of alcohol is not advisable as it may
abstract camphor from the celluloid and cause the film to become
distorted.

The Eastman twin reel machine had an upper reel joined to the lower or
developing reel, with a chain and sprockets, so that the upper reel
revolved at the same time and rate of revolution as the lower, when the
lower was being revolved at the gentle speed appropriate to the
developing process. Fans blew a draft of air over the upper reel. This
method necessitated over an hour for drying.

[Illustration:

  FIG. 117.—U. S. Air Service film developing machine for film 24
    centimeters wide.]

The Air Service model of film developing and drying machine (Fig. 117)
introduces an essential modification in the drying scheme of the Eastman
apparatus. The upper reel is quite independent of the lower reel and is
revolved at a high rate of speed, so that a whirling action is
introduced into the drying. Large rotating fans at the same time drive a
considerable volume of air across the film surface, and the combination
of the two agencies makes it possible to dry 35 feet of 18 × 24
centimeter film in 20 to 30 minutes. This for large numbers of pictures
makes the use of film even quicker than that of plates. The only
practical drawback to the apparatus is its bulk, which calls for a
separate room or trailer. This, however, seems to be inevitable in the
use of large roll film.

Cut film can be dried with speed only if placed in a draft of warm air.
Drying boxes, with a chute or chimney and with fans to drive the air
through from an alcohol stove, will dry several dozen films in an hour.
The films must not be closer together than about one inch, which makes
the drying boxes rather cumbersome.

=Marking Negatives.=—After development and drying, and before filing or
printing, each plate should be marked with data for purposes of future
identification. This is most easily done with pen and ink on the film
side (in reversed lettering) either along an edge in the unexposed
portion covered by the sheath or in a corner, so as to lose as little of
the photograph as possible. Just what data shall be inscribed is
dictated by the purpose for which the negative was made. The date,
altitude, time of day, true north (from known permanent features or from
shadow direction and time of day), number of the camera used, the focal
length of the lens. Other records, such as the plane and squadron
numbers, or even the pilot's and observer's initials, may be called for
(Fig. 75). For mapping work the scale of each of a set of negatives,
once found, may be marked, either in figures or by means of a line of
length corresponding to a fixed distance on the ground. Rectifying data
can similarly be inscribed, so that the negative can be printed in the
enlarging and rectifying camera with the minimum of delay.




                              CHAPTER XXIV
                         PRINTING AND ENLARGING


=Contact Printing.=—Single prints are made most simply in a printing
frame held at a short distance from a light source. When any quantity
must be made, as in turning out prints at high speed for distribution to
an army before an attack, _printing machines_ are employed. These
consist essentially of a light box, a printing frame of plate glass, and
a pressure pad. In the commercial models, such as the Crown and the
Ansco, which are equipped with electric light, merely bringing the
pressure pad down and clamping it automatically turns on the light,
while release of pressure terminates the exposure.

The question of regulating the distribution of light is of considerable
importance with negatives taken by focal-plane shutters of non-uniform
rate of travel. In the McIntire printer (Fig. 119), the separate
electric bulbs are on long necks in ball and socket joints, so that they
can be brought individually closer to the printing surface or farther
away from it, thus permitting a wide range of “dodging.” This printer
also has an automatic time control for the light, a valuable device
where many prints from the same negative are desired.

These machines are well suited for printing aerial negatives, either
plate or cut film, if used where a source of electric current is
available. The chief defect, which may be caused by faulty construction,
is imperfect contact between paper and negative, a cause of serious
unsharpness on prints destined for minute study in interpretation.

[Illustration:

  FIG. 118.—Printing machine.]

The printing of aerial negatives may be done either on roll or cut
paper, and if films are used, a further alternative is offered of
handling it either in the roll or in cut form. Where many prints are to
be made from one negative roll paper has some advantages, particularly
if a developing and drying machine is available. But for moderate
numbers the advantage is small, since cut prints can be developed quite
conveniently in goodly numbers in the ordinary trays. But the advantages
of keeping film in the roll form are very great, both in respect to
storage and in respect to handling during printing, as the rollers
provide the necessary tension and prevent the film “getting away.”

[Illustration:

  FIG. 119.—Interior of McIntire printer, showing lamps adjustable in
    position for “dodging.”]

[Illustration:

  FIG. 120.—Film printing machine.]

For the American Air Service, cut paper has been used exclusively. For
film printing, the Ansco machine has been equipped with roll pivots to
take film 24 centimeters wide which may be advanced in either direction
by turning large milled heads (Fig. 120). If we put rollers on the two
remaining sides of the box to handle paper we transform the printer into
the same form as a French machine, in which paper and film are moved at
right angles to each other. A disadvantage of this modification,
however, is the difficulty of examining the negative to be printed.

=Stereo Printing.=—To make separate prints from the two elements of a
stereoscopic pair and mount them side by side after proper orientation
is too slow a process if quantities of prints are needed. One method of
multiple production is to make a master stereogram, and then produce
photographic copies of it, but there is inevitable loss of quality in
this copying process. An intermediate method is to print from both
negatives on the same sheet of paper. In order to do this the negatives
must be placed in rather large frames, with mats properly located to
guide the placing of the paper. The Richard double printing frame is a
practical device which simplifies the necessary manipulations. It
consists essentially of a platform pierced with three illuminated
openings. The two negatives are compared, superposed, and orientated
over the central opening and then shifted laterally, one to each of the
two side openings, which serve both as printing frames and masks. The
printing back slides on a rod, permitting the paper to be lifted up and
moved between exposures. Once the negatives are properly placed, stereo
prints can be turned out quickly and easily.

=Enlarging.=—In the French service contact printing was the rule during
the war. The English practice, on the other hand, was to take small
negatives—4 × 5 inches, with 8 to 12 inch lenses—and enlarge them,
usually to 6½ × 8½ inches. For this purpose a regular part of the
English photo section equipment was the _enlarging camera_ (Fig. 115).
This may be briefly described as a short focus camera in which the
subject to be photographed is a negative, illuminated by transmitted
light, whose image is thrown by the camera lens on the paper or other
sensitive surface. By making the distance between negative and lens less
than that between lens and paper, the resulting print is an enlargement,
and _vice versa_. The scale of enlargement or of reduction is varied
over limits set only by the length of the camera and the amount of light
available.

The lens employed must of course possess sufficiently high quality to
preserve all the sharpness of the negative, and focussing must be done
with accuracy. Next to the lens the most important element is the light
source. This may be of the point form, such as a concentrated filament
electric lamp, an oxy-acetylene lime light, or an acetylene flame. The
latter was extensively used in the English service, while acetylene
generators for emergency purposes formed part of each American photo
truck equipment. With point light sources we must use _condensers_ to
focus the light into the projecting lens. Much less efficient, but the
only recourse where large condensers are not available, is a diffusing
glass behind the negative, illuminated either by a bank of electric
lamps with mirrors or by a U tube mercury vapor lamp, where proper
current can be got.

The device for holding the printing paper must permit quick changing,
but insure good contact. We may use either a spring plate to hold the
paper against plate glass from behind, or else a weight acting on a
lever arm of sufficient length.

The need for some automatic means of focussing an enlarging camera has
been very generally felt. An illustration of such an enlarging camera is
that put out by Williams, Brown & Earle, of Philadelphia, known as the
“Semperfocal” (Fig. 121). In this camera the movements of the lens,
paper easel and negative are so inter-related and actuated with respect
to each other that the correct focus of the instrument is maintained for
any degree of enlargement or reduction. This feature is a great help in
making up mosaic maps, where prints of continuously varying scale
ordinarily occasion serious delay for individual focussing.

[Illustration:

  FIG. 121.—“Semperfocal” enlarging camera, with mechanism for holding
    image in focus at any enlargement.]

Determining the correct enlargement for each negative of a mosaic is
perhaps the most important problem in the use of the enlarging camera
for aerial work. The correct setting of the camera may be found by
either of two methods: the negative may be previously scaled and marked
with a line on its edge, which must be projected to a definite size; or
the true location of several points in the picture as obtained from an
accurate map may be marked on the enlarging camera easel according to
the desired scale, and the negative image projected to coincide with
these. In either case, if an exact scale is desired, allowance must be
made for paper shrinkage, a matter which must be determined by previous
experiment.

=Rectifying.=—Negatives taken when the plane is not flying level will be
distorted (Figs. 134 and 135). Contact prints from these will not fit
into a mosaic, and no mere enlargement or reduction will make them
available. It is necessary with these negatives to resort to a
_rectifying camera_. This is an enlarging camera built so that the
negative and print easel may be inclined about vertical and horizontal
axes, thereby purposely introducing a distortion sufficient to offset
the distortion of the negative. Thus, if the bottom of the printing
surface is moved away from the lens, that part of the picture will be
enlarged; if moved toward the lens, reduced.

For small rectifications the common practice is to tilt the printing
surface alone, a method that is practical as long as this tilting does
not affect the focus so much as to require prohibitive stopping down of
the lens. For great distortions, such as that inherent in the principle
of the Bagley camera, it is necessary to tilt both negative and print in
order to preserve an approximate focus, a given portion of the negative
moving toward the lens as the corresponding portion of the print is
moved away. Both schemes for rectification are shown diagrammatically in
Fig. 122.

=Developing and Drying Prints.=—The developing of prints follows closely
that of cut or roll film, and so need not be treated separately.

The drying of emulsions on paper is more easily accomplished than the
drying of emulsions on glass, for two reasons: the emulsions on paper
are much more thinly coated, and there is diffusion of moisture into the
atmosphere from front and back of the printing medium. In the field a
common method has been to soak the prints in water-free alcohol and then
burn off the alcohol, thus securing a dry print within two or three
minutes after the conclusion of washing. A later method very generally
employed is to cover wooden frames three or four feet above the ground
with chicken wire or muslin, and on these lay the prints after soaking
them in alcohol. Below the frames currents of warm air rise from pans of
burning alcohol, previously used to soak the prints and now useless as
alcohol because of their high water content.

[Illustration:

  FIG. 122.—Diagram showing enlarging with and without distortion: _A_,
    enlarging without distortion; _B_, distortion for rectification of
    print, by inclining printing surface; _C_, distortion, for
    rectification of print, by inclining both negative and printing
    surface.]

Before putting them in alcohol it is advisable to squeegee all the
surface water from the prints. This may be expeditiously done by
removing them in mass from the final wash water upon a large ferrotype
plate, and either running the plate and prints together through a wash
wringer with light pressure, or covering the whole with a sheet of
blotting paper and pressing out the water underneath by means of a
rubber squeegee vigorously applied.

For base work one of the modern automatic print-drying machines used in
commercial photography would be desirable. Glossy surfaces are given
prints by the usual ferrotype plate method. But this is too
time-consuming for war practice, and besides has but doubtful advantage
where papers of the glossy type are chosen.




                                   VI
                      PRACTICAL PROBLEMS AND DATA




                              CHAPTER XXV
                                SPOTTING


“_Spotting_,” as distinct from mapping or from the photography of
continuous strips, is the photography of a definite individual
objective. In military work spotting or “pin pointing” includes the
photography of particular trenches or pivotal points in a trench system
before an attack (Fig. 123), of roads or bridges along which an advance
must pass (Fig. 124), of batteries or big guns which are the subject of
artillery fire (Fig. 125), both before and after their bombardment (Fig.
126), of gun puffs or exploding bombs (Fig. 131).

The technique of spotting consists largely in getting properly over the
target and then securing the exposure at just the right moment. This is
chiefly a question of proper piloting; but the aid which can be offered
to the pilot by camera auxiliaries designed particularly for spotting
needs is very large.

Discussion of the task of the pilot who must steer a photographic plane
accurately over a previously selected point of interest cannot be
undertaken without raising the question of who should take the picture,
pilot or observer? In the English service the most general practice was
for the pilot to be charged with the responsibility both of covering the
objective and of exposing. If a propeller drive was used on the camera,
this left to the observer only the task of changing magazines. If the
camera was hand operated the plates were changed either by the observer,
or else, as was frequently the case, distance operating devices were
attached, so that the pilot even then did everything except change the
magazines, and the observer was kept free to watch the sky for enemy
aircraft. A very desirable adjunct to the camera when plates are shifted
automatically or by the observer is a distance indicator, to show the
pilot when the shutter is set. Electrical indicators for this purpose
have been devised.

[Illustration:

  FIG. 123.—Low view of trenches on the Yser, showing concrete
    structures undamaged by bombardment.]

[Illustration:

  FIG. 124.—The Piave River. To left, destroyed bridge, to right,
    pontoon bridge under construction.

  Italian aerial photographic service.]

[Illustration:

  FIG. 125.—Showing big gun hidden in forest. (Upper left-hand corner).]

[Illustration:

  FIG. 126.—Example of spotting. Battery before and after bombardment.]

If the camera is completely hand operated, as were most of those in the
French and German services, there is little choice but for the observer
to perform the entire operation. The exposing operation could have been
delegated to the pilot, but such was not the custom with the French or
with the American squadrons using French apparatus. In this method of
operation the observer depends on the pilot to get the plane over the
target, while the pilot depends on the observer to get the picture when
the target is covered. Ample opportunity is thus offered for
misunderstanding and disagreement. This can be avoided only by excellent
sights properly aligned, for both pilot and observer, and by some means
of communication between the two men concerned.

[Illustration:

  FIG. 127.—Photograph, made with long focus lens to determine the
    results of aerial bombing. The “Tirpitz” battery of long range naval
    guns directed on Dunkirk.]

The simplest _means of communication_ is of course direct conversation.
But this is only possible in those planes, such as the DH-9, in which
pilot's and observer's cockpits are immediately together, so that, by
shouting, any desired information can be conveyed with fair ease. When
the distance is increased to four or five feet, as in the DH 4, the
loudest shouts are totally lost in the roar of the engine and the blast
of the wind. Speaking tubes and telephones are now fairly good, but are
none too comfortable or convenient to have strapped on one's head and
face. A primitive device used to some extent in the war was merely a
pair of reins attached to the pilot's arms, by which he could be
directed which way to steer. There is much to be said for a simple
semaphore system, where an indicator in the observer's cockpit actuates
a similar dial in front of the pilot, indicating “right” or “left,”
“picture obtained,” “try again,” etc. If the observer has a sight by
which he can see far enough ahead to correct the pilot's error of
pointing, the need for an accurate sight for the pilot is diminished.

[Illustration:

  FIG. 128.—Diagram showing relationship between focal length and area
    covered by plate.]

[Illustration:

  FIG. 129.—Diagram giving data on area covered at various altitudes by
    representative lens.]

[Illustration:

  FIG. 130.—Burchall Slide Rule, for calculating intervals between
    exposures, and for other aerial photographic data.]

In considering the question of sights, attention may again be called to
the poor “visibility” from the pilot's seat in the present prevailing
type of two-seater tractor plane. Blind directly in front, beneath, and
to either side (Figs. 7, 8 and 9), it is no unusual thing for a pilot to
entirely miss an objective, such as a railway line, which he can only
estimate to be beneath him by judging its distance from those objects to
either side which he can actually see. The English practice of leaving a
clear space of six inches to a foot between the fuselage and the
beginning of the wing fabric, allows the pilot to look down over the
side, a decided advantage. But for photographic purposes nothing can
compare with a good negative lens carrying fore and aft lines or wires,
so that the pilot can see his objective in ample time to head directly
for it. The lens should either be large enough so that its rear edge
gives the view directly downward, or supplemented by an additional lens
pointing directly down, so that the covering of the target is assured.
To locate such a lens in the front cockpit, free of all controls, is a
very hard task; even so its view is likely to be badly interrupted by
the landing gear. Nevertheless, so important is it, both in photography
and in bombing, to have a sight by which the plane can be accurately
directed that designers of planes should recognize this need and make
every effort to provide a suitable location.

[Illustration:

  FIG. 131.—Aerial bombardment of Trieste. Note falling bombs in center
    of picture; and exploding anti-aircraft shells over the water.

  Italian official photograph.]

[Illustration:

  FIG. 132.—Example of spotting requiring exposure at exact instant.
    Explosion following burst of bomb in ammunition dump.

  British official photograph.]

Sights for the observer have been discussed already. Here again the
negative lens is to be preferred, but while the pilot's lens needs only
directing lines in the axis of the plane (unless he takes the picture),
the observer's lens needs both an accurate center mark and an additional
upper or lower sighting point. Accurate alignment of these marks with
the camera axis must be arranged for in precise spotting.

Accurate spotting work requiring the delineation of fine detail calls
for cameras of considerable focal length. The camera of longest focal
length used in the war was the French 120 centimeter (Fig. 41). This was
employed with great success in such work as regulating the fire of heavy
railway guns brought into range only at night, to fire a few shots at
chosen angles. Photographs taken the next day would then show the exact
spot where each shell fell, and the damage it did, to serve as a guide
for the next night's operations (Fig. 127). The field of these cameras
is quite small—8 to 12 degrees—and so not only must sighting be exact
but the area covered on the ground must be accurately known. This is to
be calculated from the altitude, focal length, and plate size, by the
relation—

                   distance on ground     altitude
                   —————————————————— = ————————————
                      plate length      focal length

Data derived from such calculations may be incorporated in tables, or
graphically in diagrams such as Figs. 128 and 129.

[Illustration:

  FIG. 133.—The same subject a few minutes later. Height of smoke shown
    by shadow.

  British official photograph.]

These calculations and others required in mapping and stereo-work are
simply and quickly made by slide-rule devices. One of these, the
Burchell Photographic Slide Rule, developed in the English service, is
shown in Fig. 130. This consists of two dials, the center one of which
is mounted—usually by a pin pushed into a cork behind—so as to turn
freely, to permit its being set for altitude, focal length, ground
speed, plate size, etc., whereupon the area covered, or the appropriate
interval between exposures may be read off.

Cameras for spotting work should be capable of exposure at the exact
moment desired. For if the camera is ever to catch the gun as it
discharges, the bomb as it falls (Fig. 131), or the shell as it explodes
(Fig. 132), the photograph must be taken within the instant. Automatic
cameras, exposing at regular intervals, while adequate for mapping, are
not fitted for many kinds of spotting.




                              CHAPTER XXVI
                               MAP MAKING


=Technique of Negative Making.=—Stated in its simplest terms, the whole
problem of making a photographic map from the air consists in taking a
large number of slightly overlapping negatives, all from the same
altitude, with the plane flying uniformly level. When trimmed and
mounted in juxtaposition, or pasted together so as to overlap in their
common portions, the prints from these negatives constitute a complete
pictorial map. There is thus furnished by a few hours' labor topographic
information which would be the work of months to obtain by other means.

The making of map photographs involves all the special technique of
spotting, with much in addition. The pilot's task is not merely to go
over one object; he must navigate a narrow path, at a constant altitude,
on an even keel. If he is to make not merely a ribbon, but a map of
considerable width, he must take successive trips parallel to the first,
each displaced just far enough from the previous course to insure that
no portion is missed—a difficult task indeed.

It is the observer's duty to so time the intervals between exposures
that they overlap enough, but not so much as to be wasteful of plates or
film. He must also change magazines or films so quickly as to miss no
territory, or if some be missed, his is the task of directing the pilot
back to the point of the last exposure, where they begin a new series.

Level flying is entirely a pilot's problem. Its importance will be
realized when we consider the accompanying diagrams (Figs. 134 and 135),
where the effect on the resultant picture is shown of climbing, gliding,
or banking to either side. Prints from negatives distorted in this way
neither will be true representations of the territory photographed, nor
will they match when juxtaposed. In fact, they can be utilized only if
special rectifying apparatus is available for printing. Flying at a
constant altitude is similarly necessary if the prints are to be
utilized without enlargement or reduction in order to make them fit.

[Illustration:

  FIG. 134.—Diagram showing effect of banking on aerial photograph.]

[Illustration:

  FIG. 135.—Diagram showing effect of climbing and diving on aerial
    photograph.]

Assuming a skilled pilot who will do his part, the next step is to
calculate the exposure intervals in order to insure an adequate overlap.
If a negative lens is installed which has been marked with a rectangle
the size of the camera field, the simplest method is to estimate the
proper instant for exposure by watching the progress of objects across
the lens face. This of course requires constant attention, and it is
easier to do this only occasionally, in order to determine the ground
speed in terms of camera fields traversed per minute. Thereafter
exposures are to be made by time, as determined by a watch or clock. Any
desired degree of overlap can be chosen, and either estimated, or more
or less accurately fixed by lines marked on the negative lens at a
shorter distance apart than the edges of the field. The most usual
overlap is 20 per cent., except for stereos, which call for 50 to 75 per
cent.

In the absence of a negative lens or some other sight to show the whole
camera field, it is necessary to resort to calculation from the speed
and altitude of the plane, the focus of the lens and the dimensions of
the plate. If _A_ is the altitude, _a_ the focal length of the lens, _d_
the diameter of the plate in the direction of travel (usually the short
length is chosen for economy of flights to cover a given width), _f_ the
fractional part by which one negative is desired to overlap the next,
and _V_ the ground speed of the plane, then we have, by simple
proportion, that the interval between exposures, _t_, must be—

                                _Ad_(1 - _f_)
                          _t_ = —————————————
                                    _aV_

If _A_ = 2000 meters, _d_ = 18 centimeters, _f_ = ⅕, _a_ = 50
centimeters, and _V_ = 200 kilometers per hour, this relation gives—

                    2000 × .18 × .8 × 3600
              _t_ = —————————————————————— = 10.3 seconds
                         .5 × 200,000

The principle of overlapping map exposures is shown in the accompanying
diagram (Fig. 129), together with data calculated as above for a 4 × 5
inch plate.

It is particularly to be noted that it is the _ground speed_ of the
plane that is used. This may be calculated by knowing the air speed and
the wind velocity and direction. Fig. 136 shows the method of doing this
graphically. First an arrow is drawn representing the direction it is
desired to fly. Next a second arrow is drawn of length to represent the
wind velocity. This must be inclined toward the first arrow in the
direction of the wind, and its head is to touch the head of the first
arrow. Then with the farther end of this second arrow as a center,
describe a circle of such a length as to represent the air speed of the
plane, in the same units as the wind velocity. Connect the point where
this circle cuts the arrow of flight direction to the center of the
circle by a straight line. This line constitutes the air speed arrow,
giving the direction it is necessary to fly, at the given air speed, to
make the course desired. The length of the flight direction arrow
between its head and its point of intersection with the air speed arrow
gives the ground speed.

[Illustration:

  FIG. 136.—Diagram showing method of calculating ground speed from air
    speed and wind velocity.]

When the wind is ahead or astern this calculation reduces to the simple
subtraction or addition of the wind velocity to the air speed of the
plane. Whenever possible, mapping should be done up and down the wind
(Fig. 137). If the plane is “crabbing,” the above calculations for
overlap are only valid if the camera can be turned normal to the
direction of travel over the ground. If the camera cannot be so turned
the corners of the successive pictures overlap instead of their sides,
with quite unsatisfactory results (Fig. 138).

Calculation of the distance apart of the parallel flights necessary to
make a map of any width is done by the use of a formula similar to the
longitudinal overlap formula above, distance figuring instead of time.
Using the same symbols, and denoting the distance by _D_, we have—

                                _Ad_(1 - _f_)
                          _D_ = —————————————
                                     _a_

With the same figures as before, but substituting 24 centimeters for the
plate dimension, this relation gives—

                         2000 × .24 × .8
                   _D_ = ——————————————— = 768 meters
                               .5

[Illustration:

  FIG. 137.—Overlaps made when flying with or against the wind.]

[Illustration:

  FIG. 138.—Unsatisfactory overlaps made when plane is “crabbing.”]

It is of course largely a pilot's problem to steer the plane over
parallel courses at a given distance apart, although the observer,
noting conspicuous objects through a properly marked negative lens, may
direct the pilot by any of the means of communication already mentioned.

An alternative method of securing parallel strips, which is to be highly
recommended where enough photographically equipped airplanes are
available, is for several planes to fly side by side, maintaining their
proper separation (Fig. 139).

=Cameras and Auxiliaries for Map Making.=—Mapping can be done quite
satisfactorily by hand operated or semi-automatic cameras, provided the
observer has not too many other duties. On the other hand, the operation
of exposing at more or less definite intervals of time, irrespective of
the object immediately presented to the camera, is a largely mechanical
one. It naturally suggests the employment of an automatic mechanism,
whose speed of operation only is it necessary to watch.

If a non-automatic camera is used the timing of exposures may be done by
watching a negative lens, as described above, or by reference to a
clock, assuming that the ground speed is known through calculation. A
very practical advance over the ordinary use of a clock is to attach a
stop-watch to the shutter release, so that it is turned back to zero and
re-started at each exposure (Fig. 70). In passing, it may be noted that
if the stop-watch hand makes an electric contact which throws the
shutter release, then the device constitutes an attachment for turning
any semi-automatic camera into an automatic. The most suitable cameras
for mapping are unquestionably those of the entirely automatic type. The
use of such cameras always demands a knowledge of the ground speed. This
demand has led to many suggestions for _ground speed indicators_. The
common idea of these is to provide a moving part on the plane—either a
disc of large diameter, or a chain, or a revolving screw—whose speed may
be varied until any point upon it appears to keep in coincidence with a
point on the moving landscape below. The ground speed is then to be read
off a properly calibrated dial. Or, as a further step, the frequency of
the exposures may be directly controlled by the ground speed indicator
mechanism. The entire control of the camera would then consist merely in
occasional adjustment of the ground speed indicator.

[Illustration:

  FIG. 139.—Planes starting out to make a map by flying in parallel.]

While entirely possible in theory, these devices are not easy to work
with in practice, because the plane is always subject to some pitching
and rolling, which make it difficult to hold any object constantly on
the moving point. This is especially true at high altitudes, where the
apparent motion of the earth is quite slow compared to the swervings of
the plane. This objection is in part removed if the ground speed
indicator is carried by a gyro stabilizer.

Ordinary mapping does not demand such exquisite rendering of detail as
does trench mapping. Nor is it necessary to fly in peace-time at such
high altitudes as in war. In consequence, mapping cameras are preferably
of the short focus, wide angle type, say, 25 centimeter focus for an 18
× 24 centimeter plate. Film is to be preferred over plates because of
the greater number of exposures it is possible to make on a flight. The
shutter of the mapping camera must be extremely uniform in its rate of
travel so that the elements of the map may match in tone (Fig. 140). A
mount which permits the camera to be turned normal to the direction of
flight, such as the British turret mount (Fig. 87), is particularly
desirable if flying across the wind is necessary, as will often be the
case in mapping strips between towns or between flying fields. Devices
to indicate compass direction and altitude are called for in new and
poorly mapped territory, and may be expected to receive intensive study
in the future. The question of their utility is, however, bound up with
the whole question of the sphere of aerial photographic mapping. Up to
the present this has been almost entirely a matter of filling in details
on maps obtained by the regular surveying methods, or of making
pictorial maps for aviators. To what extent primary mapping can be done
by the airplane is yet to be determined.

[Illustration:

  FIG. 140.—A strip map, showing effect of uneven focal plane shutter
    action.]

At this point mention must be made of special cameras for securing
extremely wide angle views, thereby minimizing the number of flights.
The _Bagley camera_, devised by Major Bagley of the U. S. Engineers, is
an example. It has three lenses, a middle one pointing directly
downward, and one to either side at an angle of 35 degrees. The pictures
obtained with the side cameras are of course greatly distorted, and must
be rectified in a special rectifying camera. The resultant definition is
not good, but as the maps are made on a much smaller scale than the
original pictures, this is not a serious objection. It is a matter for
the future to decide whether the additional labor on the ground
necessary for the rectifying process is to be more expensive than the
extra flights which must be made with the ordinary types of cameras
covering a smaller angle.

=Printing and Mounting Mosaics.=—With an ordinary set of overlapping
negatives the first step toward producing a map is to _scale_ the
negatives. For this purpose one should be selected which by comparison
with a map shows no distortion, and which is on the desired scale, or is
known to have been made at the average altitude of flight. A sketch map
of the territory should then be drawn, on this scale, based on available
maps. This sketch is preferably made on a large ground glass illuminated
from behind (Fig. 141). On this all the negatives should be laid, and
their proper relative positions sought. When this is done it is evident
at once whether all the territory has been covered, and whether there
are any superfluous negatives. Each negative should then be examined as
to its scale and distortion. If it can be made to fit the scale by
simple enlargement or reduction, a line can be drawn on one edge of a
length indicating its scale. This line will later be used as a guide in
the enlarging camera. If the picture is badly distorted it must either
be replaced by another negative, or if rectifying apparatus is
available, it must be set aside for the making of a rectified print.

[Illustration:

  FIG. 141.—Scaling negatives for mosaic map-making.]

The next step is to make prints from the negatives, which may be done
either by contact, or, necessarily if differences of scale must be
compensated, in the enlarging camera. If prints to an exact scale are
required the shrinkage of the paper must be determined and allowed for.
The prints must all show the same tone, and must be uniform from edge to
edge. If the focal-plane shutter is not uniform in its travel, as is
frequently the case, this means that the print must be “dodged,” or
exposed more at one edge than the other, by locally shielding the plate
and paper during exposure. A case of the step-like effect caused by
uneven shutter action is shown in Fig. 140. The effect due to uneven
shutter action is of course absent with a between-the-lens shutter,
which constitutes a strong argument in favor of that type for use in
mapping cameras.

[Illustration:

  FIG. 142.—Arranging prints for a mosaic map.]

When the prints are made they must be mounted together on a large card
or cloth background. For a very small mosaic they may be juxtaposed by
simple examination, matching corresponding details in successive prints.
For a mosaic of any size an accurate outline map must be drawn on the
surface to which the prints are to be attached. The prints are then laid
out on this outline, moved to their correct positions, and held down by
pins (Fig. 142). When they are all arranged the final mounting may be
begun. The excess paper, beyond what is necessary for safe overlaps, may
be trimmed off, exercising judgment as to which print of each adjacent
pair is of the better quality, and utilizing it for the top one at the
overlapping junction. If one print shows serious distortion it may be
placed under its fellows on all four edges, thus minimizing its weight.
The edges are best made irregular by _tearing_. Straight edges are apt
to force themselves on one's attention in the final mosaic and give an
erroneous impression of the existence of straight roads or other
features. Both forms of edging are shown in Figs. 124 and 143.

An alternative method of securing the final print mosaic, where film
negatives are used, is to trim successive film negatives so that the
trimmed sections will exactly juxtapose, instead of overlap. The
sections are then mounted, by stickers at their edges, on a large sheet
of glass, and printed together. Captured German prints show that this
was the method commonly used with the German film camera (Fig. 62).

It will be noted that the procedure which has been described and
illustrated by Figs. 142 and 143 assumes the previous existence of a map
accurately placing at least the chief features of the country covered.
This draws attention at once to the limitations and true sphere of
aerial photographic mapping at the present time. With the cameras thus
far it is not possible, nor is it attempted, to do primary mapping of
unknown regions. Distortions due to lens, shutter, film warping and
paper shrinkage considerably exceed the figures permitted in precision
mapping. From the standpoint of geodetic accuracy the cumulative errors
of deviations in direction, altitude and level, peculiar to flying,
would soon become prohibitive.

[Illustration:

  FIG. 143.—A partly completed map. Prints mounted over an outline
    sketch map to proper scale.]

The great field for aerial photographic mapping in the near future lies
in filling in detail on maps heretofore completed as to general
outlines, or, as in the war, on maps far out of date. The war-time
procedure in country largely unknown, such as Mesopotamia, was probably
closely that which will be necessary in peace. Conspicuous points in the
landscape were first triangulated from friendly territory, and from
these the outline map was drawn, whose details were to be supplied by
aerial photographs. Much of the “mapping” of cross country aerial routes
so far done is frankly of a pictorial nature, showing conspicuous
landmarks and good landing fields—extremely valuable and useful, but not
to be confused with precision mapping. In assembling mosaics of this
kind the elaborate procedure described above is not followed. The
process is the simple one of juxtaposing adjacent prints as accurately
as possible by visual examination. Errors are of course cumulative, but
as long as exact distances are not in question this is no matter.




                             CHAPTER XXVII
                       OBLIQUE AERIAL PHOTOGRAPHY


Oblique views from the airplane are of very great value. While vertical
views are more searching in many respects, they do nevertheless present
an aspect of the earth with which ordinary human experience is
unfamiliar. Consequently they are difficult to interpret without special
training. They suffer, too, from the military standpoint, from the
limitation that it is with vertical extension just as much as with
horizontal that an army has to contend in its progress. Elevations and
depressions of land show on an oblique view where they would be entirely
missed in a vertical one. For illustration, study the picture of part of
the outskirts of Arras (Fig. 144), presenting moat, walls and
embankments, all of which would be serious obstacles, but would hardly
be noticed on a vertical view. Pictures taken from directly overhead are
eminently suited to artillery use, but oblique views of the territory to
be attacked, taken from low altitudes, formed an essential part of the
equipment of the infantry in the later stages of the war.

Pictorially, oblique views are undoubtedly the most satisfactory. The
most revealing aspect of any object is not one side or face alone, but
the view taken at an angle, showing portions of two or three sides. Best
of all is that taken to show portions of front, side and top—the
well-known but heretofore fictitious “bird's-eye view” (Fig. 145). This
possibility is ordinarily denied the surface-of-the-earth photographer,
but the proper vantage point is attained in the airplane.

Aerial obliques may be taken at any angle, although a distinction is
sometimes made between obliques of high angle and panoramic or low angle
views (Fig. 146). In addition to ordinary obliques, a very beautiful
development is the stereo oblique. Both kinds of oblique photography
call for special instrumental equipment and technique.

[Illustration:

  FIG. 144.—The outskirts of Arras. Low oblique showing contours.]

=Methods and Apparatus for Oblique Photography.=—The simplest method of
taking oblique pictures from a plane is to use a hand camera pointed at
the desired angle. Its limitations are in the size and scale of the
picture obtainable, and in the inherent limitations to the method of
camera support. A step in advance of this is to mount the camera above
the fuselage, on the machine gun ring or turret, in place of the gun.
Considerably greater rigidity is thus obtained, and heavier cameras can
be utilized, although the wind resistance is a serious factor. Excellent
obliques have been made in this way, even with 50-centimeter cameras,
but the scheme is impractical in military planes, because of the removal
of machine gun protection.

[Illustration:

  FIG. 145.—Oblique view of Capitol and Congressional Library,
    Washington.]

[Illustration:

  FIG. 146.—Fort Alvenslegen, near Metz.

  Photo by Photographic Section A. E. F.]

If the camera is fixed in the fuselage in its normal vertical position,
obliques may be and have been taken by the simple expedient of _banking_
the plane steeply. This is not to be recommended as a standard
procedure, especially for taking a consecutive series of exposures.

The most satisfactory arrangements for taking obliques are two; first,
to mount _the camera obliquely in the plane_, and second, to use a
_mirror or prism_, in front or behind the lens of the vertically mounted
camera. The first method has been employed chiefly by the French, the
latter by the English, whose gravity fed cameras could not be mounted
obliquely.

Taking up first the oblique mounting of cameras, we find two ways of
doing this: longitudinal mounting and lateral mounting. In longitudinal
mounting the camera projects forward and downward, usually from the nose
of a pusher or bi-motored plane. With this form of mounting (Fig. 147)
it is necessary of course to fly directly toward the objective. If this
is a portion of enemy trench, which must be photographed from a height
of 400 or 500 meters, the plane will be directly on top of its objective
a few seconds after the exposure is made, and be a conspicuous target,
in imminent danger of destruction. Moreover, only a single short section
of the trench would be obtained for each crossing of the line. The one
case where resort to this method is practically forced is with the
120-centimeter cameras which simply cannot be slung athwart the plane.
There is a slight advantage in this method of carrying in that the
motion of the image is less if the objective is approached, instead of
being passed at the side, and so longer exposures can be made. The
longitudinal mounting has, however, been very generally superseded by
the lateral.

[Illustration:

  FIG. 147.—120-centimeter camera mounted obliquely in the fore-and-aft
    position.]

Methods for mounting cameras obliquely for taking pictures through the
side of the plane have been discussed in detail in connection with
camera mountings and installations (Fig. 93). The chief difficulties are
want of space, obstacles at the side such as control wires and
longerons, and failure of the camera to function properly at an angle.
Even in the broad circular sectioned fuselage of the Salmson plane,
quarters are so cramped that the French 50-centimeter camera when
obliquely mounted cannot be used with the 12-plate magazine, and
recourse is made to thin flat double plate-holders. Holes in the side of
the fuselage should clear all wires and should command a view
unobstructed by the wings—which often means that the camera must be
carried behind the observer's cockpit, irrespective of the suitability
of that space from other standpoints. Cameras dependent for their action
on gravity, such as the deRam and English L type, are unsuited for
oblique suspension.

[Illustration:

  FIG. 148.—Mirror on camera cone for taking oblique views.]

For cameras which, because of their method of operation or shape cannot
be slung obliquely, the only way to take obliques is to employ mirrors
(Fig. 148) or prisms. These must be of the same optical quality as the
photographic lens. They are both necessarily of considerable weight
because they must be of large area of face to fill the entire aperture
of an aerial lens. Mirrors are lighter than prisms, but must be quite
thick to prevent distortion of the surface due to any possible strains
to their mount. Right angle glass prisms have been used by the English
with the 8 and 10 inch L cameras. The prisms were uniformly tilted to an
angle of 12½ degrees from the horizontal.

Glass mirrors can be silvered either on the rear or front surface. If on
the rear, both surfaces must be accurately parallel, which means much
greater labor and expense than if the front surface can be utilized. The
difficulty with front surface mirrors is that the metallic coating is
easily tarnished or scratched, especially if silver is used, which is
almost imperative, since all the other metals have considerably lower
reflecting powers. (Gold might serve both as mirror and color filter,
because of its yellow color.) Placing the mirror inside the camera body
in part obviates this trouble, but means the use of a special elbow lens
cone. In any case the mirror or prism occasions at least a 10 per cent.
loss of light. Pictures taken by reflectors of any kind are reversed,
and must either be printed in a camera, or on transparent film which may
be viewed from the back.

The most usual condition for making obliques is to fly very low (300 to
600 meters), with the line of sight of the camera from 12 to 45 degrees
from the horizontal. This low altitude necessitates very short
exposures, to avoid movement of the image. The picture may be taken
either the long or the short way of the plate, depending on the
character of the object and the information desired. It is to be noted
that successive oblique pictures cannot be mounted to form a continuous
panorama—this being possible with obliques only if they are taken from
one point, as from a captive balloon. If successive views are made on a
straight flight at intervals so as to exactly juxtapose in the
foreground, they overlap by a large margin the middle, and a point on
the horizon, if that shows, will be in the same position in every
picture. Mosaics of obliques could be made only by some system of
conical mounting.

=Sights for Oblique Photography.=—Any of the sights previously discussed
for vertical work, such as the tube sights, are applicable to obliques.
They must, however, be suited for mounting at an angle, in a position
convenient for the observer. In addition, provision must be made for
adjusting the angle so that the lines of sight of camera and finder are
parallel. Mounting outside the fuselage is practically the only feasible
way, and is less objectionable with oblique than with vertical sights,
as oblique sighting does not require the observer to stand up and lean
over the edge of the cockpit. Windows in the side of the fuselage,
either of celluloid or non-breakable glass, are a great aid to oblique
observation. Marks upon the transparent surface can be utilized for the
rear points of a sight of which the front point is a single fixed bead
or rectangle.




                             CHAPTER XXVIII
                    STEREOSCOPIC AERIAL PHOTOGRAPHY


One of the most striking and valuable developments in aerial photography
has been the use of stereoscopic views. Pairs of pictures, taken with a
considerable separation in their points of view and studied later by the
aid of the stereoscope, show an elevation and a solidity which are
entirely wanting in the ordinary flat aerial vista. Often, indeed, these
attributes are essential for detecting and recognizing the nature of
objects seen from above. Stereoscopic aerial photography has been justly
termed “the worst foe of camouflage.”

=Principles of Stereoscopic Vision.=—The ability to see objects in
relief is confined solely to man and to a few of the higher animals in
whom the eyes are placed side by side. When the eyes are so placed they
both see, to a large extent, the same objects in their fields of view.
Owing to the separation of the eyes the actual appearance of all objects
not too far away is different, and it is by the interpretation of these
differences that the brain gets the sensation of relief. Thus in Fig.
149 the two eyes are shown diagrammatically as looking at a cube. The
right eye sees around on the right-hand face of the cube, the left eye
on the left-hand face of the cube. The two aspects which are fused and
interpreted by the brain are shown in the lower diagram.

Stereoscopic views or stereograms, made either by photography or, in the
early days, by careful drawing, consist of pairs of pictures made of the
same object from two different points. For ordinary stereoscopic work
these points are separated by the distance between the eyes,
approximately 65 millimeters or 2¾ inches. These two pictures are then
so viewed that each eye receives its appropriate image from the proper
direction, whereupon the object delineated stands out in relief.

[Illustration:

  FIG. 149.—The principle of stereoscopic vision.]

Fusion of the two elements of the stereoscopic picture can take place
without the assistance of any instrument, if the eyes are properly
directed and focussed, but this comes only with practice. Holding the
stereogram well away from the face the eyes are directed to a distant
object above and beyond, in order to diverge the axes. Then without
converging, the eyes are dropped to the picture, which should spring
into relief. It is necessary in moving the eyes from the distant object
to the near stereogram to alter their focus somewhat, depending on how
near the stereogram is held; and the success of the attempt to fuse the
images depends on the observer's ability to maintain the eyes diverged
for a distant object while focussing for a near one. Near-sighted people
(on taking off their glasses) fuse the stereoscopic images quite easily,
since their eyes do not focus on distant objects even when diverged for
them. Transparencies are easier to fuse than paper prints, but in any
case where a stereoscope is not used the separation of image centers
should not be more than that of the eyes.

[Illustration:

  FIG. 150.—Common form of prism stereoscope.]

=Stereoscopes.=—The easier and more usual method of fusing the
stereoscopic images is by a _stereoscope_. The simplest form consists
merely of two convex lenses, one for each eye, their centers separated
by a distance somewhat greater than that between the eyes. Their
function is to bring the stereogram to focus, and, by the prismatic
action of the edges of the lenses, to converge the lines of sight which
pass through the centers of the two pictures to a point in space in
front of the observer. The two lenses should be mounted so as to provide
for the adjustment of their separation to fit different eyes and print
spacings. The most common form of stereoscope is that designed by
Holmes, for viewing paper print stereograms (Fig. 150). It has prismatic
lenses of an appropriate angle to converge pictures whose centers are
three inches apart, instead of the lesser distance appropriate to
stereograms intended for fusing without an instrument. No adjustment is
provided for varying the lens separation, but the print can be moved to
and fro for focussing.

[Illustration:

  FIG. 151.—Box stereoscope.]

[Illustration:

  FIG. 152.—Diagram of mirror stereoscope.]

Another form of stereoscope, one of the first produced, is the mirror
stereoscope (Fig. 152), now used extensively for viewing stereo X-ray
pictures. It consists of two vertical mirrors at right angles to each
other, with their edge of contact between the eyes. The two prints to be
studied are placed to right and left, an arrangement that permits the
use of prints of any size. The convergence point is controlled by the
angle between the mirrors. The Pellin stereoscope (Fig. 153) utilizes
two pairs of mirrors in a way to permit the use of large prints. The
prints are, however, placed side by side on a horizontal viewing table,
which avoids certain difficulties of illumination met with in the
simpler mirror form. The box form of stereoscope (Fig. 151) using either
prisms or simple convex lenses, is particularly adapted for viewing
transparencies, although the insertion of a door at the top provides
illumination for paper prints. The Schweissguth design (Fig. 154) is
intended primarily as an aid to selecting the portions of the prints to
be cut out for mounting. The platform on which the pictures rest is
composed of two long rectangular blocks, on which are plates of glass
raised sufficiently to permit the prints to be slid underneath. The
space between the blocks allows the unused portion of the photograph to
be turned down out of the way. Prints of any size can thus be moved
about until the proper portions for stereo mounting are found. Either
block can be moved in its own plane and also to and from the eye,
whereby two prints of somewhat different scales can be fused.

[Illustration:

  FIG. 153.—Pellin double mirror stereoscope.]

=The Taking of Aerial Stereograms.=—The normal separation of the eyes is
altogether too small to give an appearance of relief to objects as far
away as is the ground from a plane at ordinary flying heights. In order
to secure stereoscopic pairs it is therefore necessary to resort to a
method originally employed for photographing distant mountains and
clouds. This is to take the two pictures from points separated by
distances much greater than the interocular separation—by meters instead
of millimeters—corresponding to the positions of the eyes on a veritable
giant. In the airplane this is accomplished by making successive
exposures as the plane flies over the objective, at intervals to be
determined by the speed, the altitude and the amount of relief desired
(Fig. 155).

An all important question which arises immediately is: What separation
of points of view shall we select? If the exposures are too close
together there will be little relief; if too distant the relief will be
so great as to be unnatural, even offensive. Obviously we cannot here
establish a criterion of _natural_ appearance, since the natural
appearance to ordinary human eyes is devoid of relief. We may, however,
define _correct relief_ as that obtained when _the apparent height of
elevated objects is right as compared with their extension or plan_.

[Illustration:

  FIG. 154.—Schweissguth stereoscope, used for selecting portions of
    prints to be mounted.]

[Illustration:

  FIG. 155.—Method of taking stereoscopic pictures.]

In order to secure this condition it is necessary, first, that each
element of the stereoscopic pair be correct in its perspective. This is
fortunately an old photographic problem, already well understood. Its
solution is to view the photograph from a distance exactly equal to the
focal length of the camera lens. Since the normal viewing distance is
not less than 25 centimeters, lenses of this focal length at least are
requisite for correct perspective. Secondly, it is necessary for correct
relief that the two views be taken with a separation equal, on the plane
of the plate, to the separation of the eyes, or 65 millimeters. If _d_
is the interocular distance, a the viewing distance, identical with the
focal length of the lens used, and _A_ the altitude, then _D_, the
distance between exposures, is given by the relation—

                               _d_   _D_
                               ——— = ———
                               _a_   _A_

For _a_ = 25 centimeters, _D/A_ = 6.5/25, approximately ¼, or the
interval between exposures must be a quarter the altitude. With a 50
centimeter lens this becomes ⅛, and so on. These figures show the
fallacy of the suggestion sometimes made that we take stereoscopic
pictures by two cameras placed one at the extremity of each wing.

When lenses of more than 25 centimeters focal length are employed, the
stereoscope should be one capable of throwing the convergence point
farther away than the customary 25 centimeters. In the simple lens type
of instrument we can do this by bringing the centers of the lenses
closer together, and by making the focus agree with the convergence
point by adjustment of the distance between lenses and stereogram. If
enlargements are used they should be treated in all respects as
originals made by lenses of the greater foci corresponding to the scale
of the enlargement.

When all the conditions are covered, the appearance presented in the
stereoscope is that of a _model_ of the original object at a distance
_a_, and _a/A_ times natural size. If pictures are made at exposure
intervals less than those indicated for correct relief, they show
insufficient relief. This does not, however, give an unnatural effect,
because anything between no relief and “correct” relief appears natural
with large objects which are not ordinarily seen in relief by eyes not
Brobdignagian. Conversely, stereograms made with too large exposure
intervals show exaggerated relief. Yet this is often no objection. It is
indeed rather an advantage if we wish to bring objects of interest to
notice. Consequently, so long as the exaggeration of relief is not
offensive, the permissible limits of exposure interval are pretty large.
Actually, the eye tolerates such great deviations from strictly normal
conditions that satisfactory stereoscopic effects are obtained for
pictures viewed at very different distances from the focal length of the
taking lens, and with the axes of the eyes parallel or even diverging,
although there is some strain whenever focus and convergence points
differ. On the whole, therefore, it may be said that the conditions
above laid down for correct relief are only a normal, to be approximated
as nearly as is practicable.

Having established the correct relation of taking points for stereos the
next problem is how to determine these when in the plane. The simplest
way is by means of a _stereoscopic sight_. This consists essentially of
two lines of sight (fixed by beads, crosses, or other objects), inclined
toward each other at the angle determined by the ratio of the ocular
separation to the focal length of the lens. If the back sight is made a
single bead or cross, the rest of the stereo sight will consist of two
beads or crosses, separated from each other by the ocular distance of 65
millimeters, and distant from the back sight by the focal length of the
lens (Fig. 157). The first picture is taken when the object is in line
with the forward pointing line of sight, the second when it lies along
the backward pointing one. Like other sights, the stereoscopic sight may
be attached either to the camera, or if this is fixed in position, to
any convenient part of the plane. A very simple sight for vertical
stereoscopic photography consists of an inverted V painted on the side
of the fuselage, so that the eye can be placed at the vertex and sighted
along either leg.

The common method of determining the space between exposures is by the
_time_ interval. If _V_ is the speed of the plane, and _t_ the desired
time interval, we have, from the last equation—

                                  _D_   _dA_
                            _t_ = ——— = ————
                                  _V_   _aV_

If _A_ = 2000 meters, _d_ = 65 millimeters, and _a_ = 25 centimeters,
and if the plane is traveling 200 kilometers per hour, the time interval
must be—

                    .065 × 2000 × 3600
                    —————————————————— = 9.4 seconds
                      .25 × 200,000

At 1000 meters altitude the interval will be half this, and so on in
proportion. If the pictures are taken with a 50 centimeter focus camera,
and are hence to be viewed at 50 centimeters convergence distance
instead of at 25, the time will again be halved. These relations are
clearly shown in the diagram (Fig. 156). Here the left-hand portion
shows how to find the stereoscopic base line at each altitude for each
focal length; while the right-hand portion shows how to translate this
into time interval for any plane velocity. The Burchall slide rule (Fig.
130) shows another way to arrange these data in form for rapid
calculation.

[Illustration:

  FIG. 156.—Chart for calculating intervals between exposures for
    stereoscopic pictures.]

Plates used for stereoscopic negatives should be at least twice as long
as the ocular separation, if correct relief is desired, and the full
size of the stereoscope field is to be utilized. This relation follows
at once if we consider that we wish to cut from each negative a
rectangle 65 millimeters wide, and that the image of the target has
shifted 65 millimeters between exposures. If the plate is larger than
this there is opportunity to select the view, or to pick several. If the
plate is smaller the elements of the stereogram must be narrow strips.
This, however, holds only for contact prints.

The ordinary English practice in making stereo negatives is to take
successive pictures with an overlap of 60 to 75 per cent. This practice
is probably dictated by the 4 × 5 inch plate, since 60 per cent. overlap
on 4 inches means a separation of just over an inch and a half instead
of 2¾, but it leaves 2½ inches of picture common to the two negatives.
With ¾ overlap the common portion is 3 inches, which permits of cutting
2¾ inch prints, and allows some latitude for irregular motion of the
plane or for chance error in calculation of intervals. Data on the basis
of ¾ overlaps for a 4-inch plate are shown in connection with Fig. 155
which shows in diagrammatic form the variation of exposure interval with
height, together with other points of interest.

=Elevation Possible to Detect in Stereoscopic Views.=—Can the actual
difference in elevation be discovered by the use of stereoscopic views?
An approximate idea may be obtained from the following considerations:
Suppose we have two small point-like objects, one above the other, such
as a street lamp globe and the base of the lamp pillar. In a view taken
from directly overhead these will be superposed, and so will not be
capable of separation. But, as the point of view is shifted sideways,
the two objects separate, until a point is reached where they can just
be distinguished as double. When this condition holds for either picture
of the stereoscopic pair it will be possible to obtain stereoscopic
relief.

Now the separation which can just be distinguished is commonly assumed
to be one minute of arc. This angle corresponds to about 1/3400 the
distance from the eye to the object. If the object is assumed at a
distance _a_ from the face, and on a line with one of the eyes, which
are separated by the distance _d_, then (all angles being small) the
object must be of height _a/d_ times the horizontal distance which
corresponds to one minute. For 25 centimeters' viewing-distance this
quantity is about 4, so that the least perceptible elevation is 4/3400
or about 1/900. The stereogram having been made under conditions
giving correct relief, this fraction is also the fraction of the
altitude of the plane when the photograph was taken which may be
detected. An object as high as a man (6 feet) should be visible as a
projection in a stereoscopic view taken at 6 × 900 = 5400 feet. This
relation—1/900—holds (irrespective of the focal length of the lens),
as long as the conditions for correct relief are maintained.

=Stereoscopic Aerial Cameras.=—Cameras for aerial stereoscopic
photography need in no way differ in construction from those made for
mapping or spotting, provided only they permit exposures to be made at
short enough intervals. The addition of special sights, as already
discussed, constitutes the only real difference between single view and
stereoscopic aerial cameras. But even without such sights ordinary
aerial cameras are applicable to stereo work by the usual procedure of
determining the exposure spacing by time.

One scheme employed for taking low stereos, where the interval is only
two or three seconds, is to mount two cameras in the plane, exposing
them one after the other at the correct interval. Another method which
has been tried with success is the use of a double focal-plane shutter
in a single lens camera (Fig. 157). The two shutters are side by side,
with their slots parallel to the line of flight. To take a stereo
negative we expose first the shutter nearer the tail of the plane, and
then the other, after an interval which can be calculated from the speed
and altitude, or, better, determined by a stereoscopic sight. The two
views are thus obtained on a single plate. Prints from these negatives
are transposed right and left, and, if the prints are viewed in an
ordinary stereoscope, have to be cut apart and transposed for mounting,
or else this may be done to the negatives.

[Illustration:

  FIG. 157.—Aerial hand camera fitted with two complimentary shutter
    slits and double sight, for stereoscopic photography.]

In this connection attention may be drawn to an alternative method of
viewing stereograms, which may be used on transposed prints—a method
which needs no instrument, and so has sufficient advantage to even
warrant mounting ordinary stereoscopic pairs in the transposed position
for observation. This method consists in crossing the optic axes, in the
fashion illustrated in Fig. 158. A finger is held in front of the face
in such a position that the left stereogram element and the finger are
seen in line by the right eye; the right element and the finger by the
left eye. The proper position is found by alternately closing each eye,
and advancing or retracting the finger. Then both eyes are opened and
converged on the finger tip, which is thereupon dropped, leaving the
picture standing out in relief. An opportunity to try this method is
afforded by Fig. 159.

[Illustration:

  FIG. 158.—Method of fusing transposed stereoscopic images by crossing
    the optic axes.]

=Stereo Obliques.=—The theory of making oblique stereo pictures is
identical with that of other stereos. The only problem peculiar to
obliques is that of making the exposures at short enough intervals
apart. This problem is due largely to the fact that oblique views are
ordinarily taken from low altitudes, for the purpose of “spotting”
particular objects, rather than for mapping the gross features of an
extended area. The same problem of how to secure a short exposure
interval is met with when we attempt to take vertical stereos from a low
altitude, but as already discussed, it is much preferable from the
pictorial standpoint that pictures of definite small objectives be made
obliquely.

Another reason for taking stereo obliques from points but little
separated is of some interest in connection with the discussion above
given of “correct” and “natural” relief. When the relief is “correct”
the object appears, as already stated, to be a small model in its true
proportions, standing at the convergence distance. When the eyes are
converged to a small object 25 to 50 centimeters away all objects beyond
are hopelessly transposed and confused. This does not happen when we
look at large distant objects, since their background is at a distance
effectively but little beyond them. As a result, when a stereo oblique
is made in “correct” relief of such an object as the Washington monument
with buildings beyond, the confusion of the background presents an
appearance entirely contrary to our visual experience with objects as
large as the neighboring buildings are known to be. This effect may be
avoided by choosing a uniform background such as grass, or by taking the
pictures very much closer together, at the expense of “correct” but at a
gain in “natural” relief.

Stereo obliques can of course only be made with any facility by
laterally pointing cameras. From the calculations already given it
appears that a “correct” stereo oblique of an object 500 meters away
will mean exposures only two or three seconds apart, too short an
interval for any of the ordinary plate-changing and shutter-setting
mechanisms; and the case is even worse should less relief be desired.
One solution of this problem has been the use, already mentioned, of two
cameras mounted together, either side by side or one over the other,
with separate shutter releases. Both releases may be controlled by the
observer, using a sight, or else pilot and observer may work in harmony
as has been recommended in the English service, where the pilot releases
one shutter and the observer counts time from the instant he sees the
first shutter unwind and releases the second.

[Illustration:

  FIG. 159.—Oblique stereogram made with stereoscopic aerial camera
    (Fig. 157). To be viewed by crossing the optic axes (Fig. 158).]

A very satisfactory apparatus for the taking of stereo obliques consists
of a 10-inch focus hand-held camera (Fig. 157), provided with a
two-aperture focal-plane shutter. The right-hand half of one curtain
aperture is blocked out, the left-hand half of the other. The first
pressure on the exposing lever exposes one-half of the plate, the second
the other. A stereoscopic sight of the type already described is placed
on the bottom. To make an oblique stereo negative the camera is held
rigidly by resting the elbows on the top of the fuselage and the first
exposure is made when the object comes in line with the rear sight and
the leading front sight. The eye is then moved so as to look along the
line of the rear sight and the following front sight, and when the
object is again in alinement the second pressure is given the exposing
lever. Fig. 159 shows a stereo oblique made by this camera. The elements
are transposed right and left, and the stereogram may be viewed by
crossing the optic axes as shown in Fig. 158, or the two pictures may be
cut apart and remounted.

=The Mounting of Aerial Stereograms.=—The first step in making the
printed stereogram is to select two pictures taken on the same scale,
but from slightly different positions. These may be two chosen from a
collection made for other purposes, or else a pair taken at distances
calculated to fit them for stereoscopic use. The next step is to mark
the center of each picture, either with easily removed chalk or with a
pin point. They are then superposed, and afterward carefully moved apart
by a motion parallel to the line joining their centers when superposed.
The final step before mounting is to mark out and cut the two elements,
their bases being parallel to the line of centers, their horizontal
length the distance between the optic axes of the stereoscope (or as
near this as the size of the prints will permit). They are then mounted
on a card, with their centers separated by approximately 65 millimeters.
The right-hand view is the one showing more of the right-hand side of
objects, and vice versa. This process of arranging, cutting, and
mounting is shown clearly in Fig. 160. In this case the stereoscopic
elements lie symmetrically about the line joining the centers of the
original prints. This is not necessary, as they may be selected from
above or below this line so long as their bases are parallel to it. A
simplification of this method consists in superposing the two prints,
laying over them a square of glass of the size to which they are to be
cut, then turning it so that a side is parallel to the line of centers,
and cutting around it through both prints with a sharp knife. The
principle and results are of course the same with both methods.

[Illustration:

  FIG. 160.—Method of arranging stereoscopic prints for cutting.]

If large numbers of stereoscopic prints are required it is necessary,
for economy of time, either to photograph a finished stereogram and make
prints from this copy negative, or to set up special printing machines.
Under the general discussion of printing devices a stereoscopic printer
is described (the Richard) in which the two negatives are placed so that
stereo prints can be got by two successive printings on one sheet of
paper.

=Uses of Stereoscopic Aerial Views.=—Attention has already been called
to the characteristic flatness of the aerial view. Neither the picture
on the retina nor that on the photographic plate affords any adequate
idea of hills and hollows. Unless shadows are well defined, small local
elevations and depressions cannot be distinguished from mere difference
in color or marking. Even in the presence of shadows it is often only by
close study that differences of contour are noticeable. But with
stereoscopic views these features stand out in a striking manner. Taking
our illustrations from military sources, we may note the use of
stereoscopic pictures to detect undulations of ground in front of
trenches (Fig. 161). They reveal the hillocks, pits, small quarries,
streams flowing behind high banks, and other features which make the
attack hard or easy. Commanding positions are shown, the boundaries of
areas exposed to machine-gun fire, and the defilades where the attackers
may pause to reform. Concrete “pill boxes” are located in the midst of
shell holes of the same size and outline, and can be differentiated from
them.

Railway or road embankments and cuts can be detected and studied to
extraordinary advantage in stereoscopic pictures. Thus what appears to
be a mine crater on a level road, easily driven around, may be a gap
blown in an embankment, a serious obstacle indeed. Bridges, observation
towers and other elevated structures jump into view in the stereoscope
when often they have entirely eluded notice in the ordinary flat
picture. Once presented in relief, camouflaged buildings or gun
emplacements, however carefully painted, are ridiculously easy to pick
out.

[Illustration:

  FIG. 161.—Typical stereogram of military detail. Fuse by looking at a
    distant object over the top of the page, and quickly dropping the
    eyes to the print.]

Practical peace-time applications of stereoscopic views can easily be
foreseen following the lines of war experience. Such, for instance,
would be the study of proposed railway or canal routes. A series of
stereograms would obviate the necessity of contour surveys, at least
until the exact route was picked and construction work ready to start.

Apart from their utilitarian side, however, stereoscopic views have very
great pictorial merit. Stereoscopic pictures of cathedrals, public and
other large buildings, have often great beauty, and afford opportunities
for the study of form given by no other kind of representation, short of
expensive scale models. They may very well lead in the near future to a
revival of the popularity of the stereoscope.

=Impression of Relief Produced by Motion.=—An appearance of solidity can
be obtained in _moving pictures_ by the simple expedient of slowly
moving the camera laterally as the pictures are taken. As an
illustration, if the moving picture camera is carried on a boat while
structures on the shore are photographed, when these are projected on
the screen they appear in relief, due to the relative motion of
foreground and background. As relief of this sort is not dependent on
the use of the two eyes, it demands no special viewing apparatus. This
idea has been utilized to a limited extent in ordinary moving picture
photography by introducing a slow to-and-fro motion of the camera, but
this can hardly be considered satisfactory, since this motion is so
obviously unnatural.

In moving pictures made from the airplane the normal rapid motion of the
point of view is ideal for the production of the impression of relief in
the manner just described. For instance, in moving pictures of a city
made from a low flying plane, the skyscrapers and spires as they sweep
past stand forth from their more slowly moving background in bold and
satisfying solidity. In fact, such pictures probably constitute the most
satisfactory solution yet found of the vexing problem of “stereoscopic”
projection. No better medium can be imagined for the travel lecturer to
introduce his audience to a foreign city than to throw upon his screen a
film made in a plane approaching from afar and then circling the
architectural landmarks at low altitudes.




                              CHAPTER XXIX
                THE INTERPRETATION OF AERIAL PHOTOGRAPHS


Oblique aerial photographs if on a large enough scale are even easier to
interpret than are ordinary photographs taken from the ground, since
they practically preserve the usual view, and add to it the essentials
of a plan. With verticals, however, this is far from the case. In them
all natural objects present an appearance quite foreign to the ordinary
mortal's previous experience of them. This may be easily demonstrated by
taking any aerial view containing a fair amount of detail and trying
systematically to identify each object. A necessary preliminary to doing
this accurately is acquaintance with and study of the ground
photographed, or of similar regions, and of objects of the same
character as those likely to be included.

The interpretation of military aerial photographs is of such importance,
and has become such an art, that it is the function of special
departments of the intelligence service. Extended courses in the subject
are now given in military schools. This instruction must cover more than
the interpretation of aerial photographs as such. General military
knowledge is essential, so that not only may photographed objects be
recognized, but the significance of their appearance be realized.
Whether attack or retreat is indicated; whether a long range bombardment
is in preparation, or a mere strengthening of local defences.

The natural difficulties of interpreting aerial views are enormously
increased by the unfamiliar nature and frequently changed character of
the military structures, and particularly by the attempts made to
conceal these from aerial observation by selection of surroundings and
by camouflage. The small scale of the photographs, in which a machine
gun shows as a mere pin point, adds to the uncertainty, with the net
result of making interpretation a task of minute study and deduction
worthy of a Sherlock Holmes.

Little detailed information on interpretation can be profitably written
in a general treatise, partly because the illustrations available are of
a highly technical military character, partly because original
photographs instead of halftone reproductions are practically imperative
for purposes of study. Nevertheless some general instructions,
applicable to any problem of interpretation, may be given, as well as a
few illustrations, drawn from military sources, which will serve to show
the detective skill necessary.

First of all it is important that the print or transparency be held in
the right position. The shadows must always fall toward the observer;
otherwise, reliefs will appear as hollows and hollows will show as
hills. The reason for this is that the body ordinarily acts as a shield,
preventing the formation of shadows except by light falling toward the
beholder. Thus in Fig. 162 the slag heap looks like a quarry when the
shadows fall away from one. The necessity for proper direction of
shadows is, it may be noted, in conflict with the ordinary convention
for the orientation of maps—at least in the northern hemisphere. A city
map, made by sunlight falling from the south, presents its shadows as
falling away from the observer, when it is mounted with its north point
at the top, as is customary. As a consequence buildings in aerial
photographic mosaics of cities occasionally look sunken instead of
standing out.

[Illustration]

   Wrong way. Shadows falling away    Right way. Shadows falling toward
           from observer.                         observer.

   FIG. 162.—The wrong way and the right way to hold a photograph for
                            interpretation.

[Illustration:

  FIG. 163.—Guide to interpretation of trench details.]

The relation between the shape of the shadow and the object casting it
must be well learned. This is a part of the training of every
architectural draftsman, but the appearance of shadows from above has
not heretofore been a matter of importance. The difference between high
and low trenches, between cuttings and embankments, between shell holes,
occupied or unoccupied, and “pill boxes,” must be detected largely from
the character of the shadows. Which elevations and depressions are of
military and which of merely accessory nature, whether this black dot is
a machine gun or a signaling device, whether that dark spot is an active
gun port or an abandoned one—these are all matters of shadow and of
light and shade study. Several illustrations of these points appear in
Figs. 163, 164 and 165.

[Illustration:

  FIG. 164.—Guide to interpretation of shell holes and other pits.]

[Illustration:

  FIG. 165.—Illustrating the importance of distinguishing between
    objects of similar appearance but different military importance.]

Shadows may be used to get exact information as to directions and
magnitudes. If we know the time of day at which a picture is taken, the
direction of the shadows will give the points of the compass. A chart
for doing this is shown in Fig. 166. The length of a shadow is a measure
of the height of the object casting it, and the exact relation between
the two dimensions is determined by the day and hour. Fig. 167 embodies
in chart form the values of this relationship for all times of the year
and day, while Fig. 168 shows the kind of picture in which shadow data
could be utilized to great profit.

[Illustration:

  FIG. 166.—Location of true north from direction of shadows. Place the
    dial on the photograph, the hour line corresponding to the time it
    was taken being pointed in the direction of the shadows. North lies
    between the two arrows, the exact direction being obtained by
    joining the center of the dial to the point on the figure of eight
    corresponding to the date on which the picture was taken. (Number on
    figure of eight represent the 1st of the month.)]

Minute changes, both in light and shade and in position, must be watched
for with great care. Naturally growing foliage and the cut branches used
for camouflage differ in color progressively with the drying up of the
leaves. Hence a mere spot of lighter tone in a picture of a forest,
especially if the picture is taken through a deep filter, becomes
instant object for suspicion. The complete study of any position calls
for photographs of all kinds—verticals, obliques, and stereos.
Stereoscopic views are the worst foe to camouflage. A bridge painted to
look like the river beneath is labor thrown away if the stereo shows it
to be a good ten feet above the real river!

[Illustration:

  FIG. 167.—Length of shadow of object one meter high, at different
    times of the day and year, for latitude of Paris.]

[Illustration:

  FIG. 168.—Bethune, August, 1918, illustrative of interpretation by
    shadows.]

[Illustration:

  FIG. 169.—Typical trench photograph showing first and second lines,
    communicating trenches, listening posts, machine gun emplacements,
    and barbed wire.]

A few illustrations of the more ordinary and obvious objects whose
detection is the subject of aerial photography are shown in accompanying
figures. Fig. 169 pictures a typical trench system, with barbed wire.
The trenches show as narrow castellated lines, from which run the zigzag
lines of communicating trenches, saps, and listening posts. The minute
pockmarks behind the main trench lines are shell holes and machine gun
pits. The barbed wire shows as double and triple gray bands, intricately
criss-crossed at strategic points. Another form of defence, intended for
the same purpose as the barbed wire of the western front, is that
furnished by overthrown trees in forest regions. Fig. 170 reveals a
mountain fortress surrounded by a zone of felled trees, and indicates in
striking manner the value of the information a single aerial photograph
may furnish to an attacking force. Fig. 123 shows on a comparatively
large scale opposing trench systems in which a natural obstacle—a
river—separates the adversaries. Nicks and dots indicate machine guns to
the skilled eye, and several rectangular structures are revealed as
concrete buildings which have survived unscathed the shell fire which
has obliterated, and caused to be rebuilt, nearly every other element of
the trench system.

[Illustration:

  FIG. 170.—A mountain fort surrounded by felled timber.]

Isolated battery emplacements (Fig. 171) must be carefully studied to
learn if they are in use. The chief indication is given by the paths the
men make in going and coming; these show as fine light lines,
obliterated by growing vegetation if long disused. Another indication is
the blast marks in front of the gun muzzles; occasionally the sensitive
plate will catch the actual puff of smoke as the gun is discharged.

[Illustration:

  FIG. 171.—Three stages in the life of a battery.]

Railways of various gauges show as thin lines, crossed by ties, and
exhibiting the characteristic curves and switches. They are particularly
important to detect because they naturally lead to guns or supplies of
importance. Abandoned railways from which the rails and ties have been
removed leave their marks on the ground and must be carefully
distinguished from lines in actual operation.

Aviation fields are easily recognized by the hangars, often with “funk
hole” trenches alongside for the men to take shelter in during air raids
(Fig. 172). Other characteristic features are the “T” which shows the
direction of the wind to the returning pilot, and of course the planes
themselves, standing on the ground. But the field may be inactive, and
the planes merely canvas dummies, so that to pierce the disguise, all
paths, ruts, and other indications of activity must be minutely studied.

Overhead telegraph and telephone lines are revealed when new by a series
of light points (Fig. 174), where the posts have been erected in the
fresh turned earth. Later, when the fields through which they pass are
cultivated, the post bases show as islands left unturned by the plow. In
winter the wires reveal their position by black lines in the snow caused
by drippings. Buried cables are indicated while building by their
trenches, and for some time afterward by the comparatively straight line
of disturbed earth.

[Illustration:

  FIG. 172.—Aviation field, showing hangars, planes, landing “T” and
    refuge trench.]

Just as the detective of classic story makes full use of freshly fallen
snow to identify the footprints of the criminal, so does the aerial
photographer utilize a snowfall to pierce the enemy's attempts at
deception. Tracks in the snow show which trenches or batteries are in
actual use. Melting of the snow in certain places may mean fires in
dugouts beneath. Black smudges in front of trench walls show where guns
are active. Guns, wire and other objects, however carefully painted to
match the gray-green earth, stand out in violent contrast to this new
white background (Fig. 173).

[Illustration:

  FIG. 173.—Trenches and barbed wire in the snow of an Alpine ridge.
    Italian Air Service photograph.]

[Illustration:

  FIG. 174.—A fully interpreted aerial photograph.]

After the aerial photograph has been interpreted the results of the
interpretation must be made available to the artilleryman or the
attacking infantryman. This may be done by legends marked directly on
the photograph. Another method is to mount over the photograph a thin
tissue paper or oilskin leaf, with the interpretation marked on it. A
yet more elegant method consists in outlining all the chief features of
the photograph in ink, writing in the points of importance in
interpretation, and then bleaching out the photograph with potassium
permanganate solution. Photographic copies of the resultant line drawing
are then mounted side by side with the original photograph. Fig. 174,
which shows a fully interpreted photograph, is an example of this kind
of mounting.




                              CHAPTER XXX
                        NAVAL AERIAL PHOTOGRAPHY


The problems of naval aerial photography are quite different from those
of military aerial work, and on the whole they are more simple. At the
same time, photography has played a considerably less important part in
naval aerial warfare than in land operations. Photography as a necessary
preparation for attack has not figured in naval practice nearly so much
as have the record and instruction aspects. To some extent this is due
to the nature of the naval operations in the Great War, to some extent
to the limitations of ceiling and cruising radius of the naval aircraft.

A photographic reconnaissance, preceding and following a bombardment of
shore batteries; a photographic record of the ships at anchor, as at
Santiago; a photograph of the forts defending a channel, as at Manila;
photographs, quickly developed and printed, of an approaching fleet—all
these are possibilities of great usefulness in naval warfare between
contestants both of whom “come out” and carry the struggle to the
enemy's gates. But in the recent war the use of the submarine,
operations under cover of fog, the striving for “low visibility,” and
the considerable distances to be traversed to reach the enemy lairs,
have conspired to limit the development of photography as a major aid to
naval combat. Probably when the whole history of the conflict is told we
shall learn that the Zeppelins which cruised over the North Sea, keeping
the Allied fleet under observation, had a regular routine of
photographic work. In the Italian zone, where much of the enemy
territory and several important naval centers lay at only short
distances over the Adriatic, the naval photographic service more nearly
rivalled that of the army than in the English, French and American zone
of activity in the Channel and North Sea.

[Illustration:

  FIG. 175.—A lighthouse, as the naval flier sees it.]

[Illustration:

  FIG. 176.—A threatened submarine attack. Throwing out a smoke screen
    to protect a convoy. British official photograph.]

The majority of the photographs made in the British service were
obliques, taken by short focus (6 to 10 inch) hand-held cameras. This
type was employed partly because of difficulties to be noted presently,
in using other forms of cameras, but more especially because such
pictures sufficed for the kind of information desired. A hand-held
camera formed part of the outfit of each flying boat and dirigible, but,
unlike land reconnaissance, planes ascending primarily for picture
taking were unknown in their naval service. Usually no photographic
objective was predetermined—photographs were made only if objects of
interest were come upon. Mapping also formed no part of the seaplane's
work. Four plates would be carried, instead of as many dozens in the
land machine, and often these would come back unexposed. There were of
course some photographic flights planned out beforehand, for the purpose
of photographing lighthouses and other landmarks whose appearance from
the air should be known to the naval aviator (Fig. 175). Among the
accidental and record types of photograph come convoys (Fig. 176), whose
composition and arrangement were made a matter of record, particularly
if any ship was out of its assigned position. Photographs of oil spots
on the sea surface, or other results of bomb dropping, were necessary
evidence to establish the sinking of a submarine (Fig. 179). Pictures of
all types of ships friendly, neutral, and where possible, enemy—were a
much needed part of naval equipment, in particular pictures of friendly
destroyers and submarines, which should not be bombed by mistake. For
safe navigation it was essential to have photographs of uncharted wrecks
(Fig. 181), of buoys out of place and of ships failing to return signals
or otherwise to comply with rules. The great majority of the pictures
were taken from altitudes of not more than 300 meters.

[Illustration:

  FIG. 177.—Submarine coming to the surface.

  U. S. Naval Air Service photograph.]

[Illustration:

  FIG. 178.—Dropping depth bombs.]

[Illustration:

  FIG. 179.—The submarine destroyed. Destroyer on tell-tale oil patch.
    British official photographs.]

[Illustration:

  FIG. 180.—A convoy at anchor in port.]

Hand-held cameras for naval work have practically the same design as
those for land work. In view of the smaller number of pictures taken on
naval trips, and the consequent absence of any need for great speed in
changing plates, the ordinary two-plate dark slide has been found
satisfactory in the English service. But these are much less convenient
than the bag magazines used in the U. S. Naval hand camera (Fig. 31).
The sights on the naval hand camera are preferably of the rectangular,
field indicating type, especially useful in photographing extended
objects such as convoys. As the flying boat travels comparatively slow,
it is easy for the observer to stand up to take pictures, and the sight
is conveniently placed on top. But if held out over the side for
verticals the sight must be on the bottom. Rectangular sights in both
positions are provided in the English camera (Fig. 186). Naval cameras
should be immune from moisture, which means doing away with all wooden
slides or grooves. A praiseworthy practice is to carry the camera in a
waterproof bag.

[Illustration:

  FIG. 181.—Airplane photography as an aid to salvaging. Position of
    wrecked merchantman twelve fathoms down revealed by photograph from
    the air.

  Photograph by British Air Service.]

[Illustration:

  FIG. 182.—A sea plane.]

[Illustration:

  FIG. 183.—A flying boat.]

[Illustration:

  FIG. 184.—A dirigible or “blimp”—possibly the photographic aircraft of
    the future.]

[Illustration:

  FIG. 185.—English “Type 18” hand camera on bracket for exposing
    through side window of flying boat.

  British official photograph.]

[Illustration:

  FIG. 186.—Camera mounted in bracket from forward cockpit of flying
    boat.

  British official photograph.]

Cameras other than of the hand-held form have been little used in sea
planes, owing to the difficulties of installation. The hydro-airplane,
consisting of an ordinary airplane fuselage mounted on two pontoons
(Fig. 182), can carry the same kind of photographic equipment as the
land machine. But if it has a single central pontoon this is not
feasible. The hydro-airplane is, however, largely superseded by the
flying boat (Fig. 183), whose fuselage, of boat form, rests directly on
the water. In this type of sea plane, views taken vertically downward
are not easy to make. In the larger flying boats the hull projects out
horizontally a matter of several feet beyond the side of the cockpit. An
ordinary outboard mounting is therefore out of the question. The camera
must either be held out at arm's length or else mounted on a long
bracket (Fig. 186). The usual place for carrying the camera is in the
front cockpit with its magnificent all-round view. Obliques can, too, be
taken in great comfort from the side windows behind the wings, as shown
in Fig. 185. The possibility of cutting a hole in the bottom of a flying
boat to take care of a vertical camera is not entertained in British and
American naval circles. Nevertheless it is the regular practice in the
Italian service, with their small high ceilinged flying boats. In them a
round hole is cut in the floor, stopped with a plug and rubber gasket.
After the boat rises into the air the hole is opened, and the regulation
Italian camera is set securely in a frame on the floor over the hole
(Fig. 187). Photographs are taken to the capacity of the camera, and if
it is desired another camera is put in its place, till all its plates
have been exposed, and then even a third. Before coming down the hole
must of course be closed again. Sliding doors have been designed to
close this aperture, but have not proved sufficiently water-tight,
although such a device could undoubtedly be worked out.

[Illustration:

  FIG. 187.—Italian flying boat with camera mounted on the floor.]

With its space for five or more passengers, and with its low speed, the
modern flying boat affords an excellent craft for photographic work.
There is ample room for any size of camera, and for any style of
mounting, if we assume that there is no objection to an opening in the
bottom. The low ceiling of these ships, however, prevents their use for
certain forms of aerial photography which should be of the greatest
importance. Operations against shore stations—harbors, docks, shipyards,
ships at anchor, and fortifications—cannot be undertaken for fear of
anti-aircraft guns and hostile land planes. The solution of the problem
of carrying and launching fast high flying planes from ships will
immediately extend the usefulness of aerial photography to coastal work.
In the recent war, such of this as was done, along the Belgian coast—the
shore batteries, and the results of naval operations at Zeebrugge and
Ostend—was done by land planes from territory held by the Allies. The
photographic equipment of sea planes of the type suggested will of
course present special problems, but the apparatus used will be apt to
approximate closely to that of the land planes.




                                  VII
                    THE FUTURE OF AERIAL PHOTOGRAPHY




                              CHAPTER XXXI
              FUTURE DEVELOPMENTS IN APPARATUS AND METHODS


Prophecy is an undertaking that always involves risk. The prophet's
guess of what the future will bring forth is based only on the
tendencies of the past, the most urgent needs of the present, and the
activity of his imagination. He may easily—and he usually does—entirely
overlook certain possibilities which may arise apparently from nowhere
and which profoundly affect the whole trend of development. Conditions
which dominate at the present time—such as military necessity—may
happily drop into the background and free the science from some of its
severest restrictions. With this caution, some future possibilities in
apparatus and methods may be presented along the lines already used in
discussing the present status of aerial photography.

=Lenses.=—From the military standpoint the next steps in lens design
would be toward telephoto lenses on the one hand, and on the other
toward lenses of short focus and wide angle. The telephoto lenses used
for spotting would be of long equivalent focus—a meter and more—but of
handy size, that is, not more than 50 centimeters over all working
distance. The wide angle short focus lenses would be designed for low
flying reconnaissance or quick mapping work. They would also be demanded
for peace-time mapping projects, where the largest possible amount of
territory should be covered in a single flight. Both types of lens
should be pushed to the extreme in aperture, for short exposures and the
maximum of working days will always be demanded.

=Cameras.=—Peace-times will give the necessary opportunity to develop
self-contained and therefore simply installed cameras. They will at the
same time be made very completely automatic but simple to operate in
spite of their complexity. Such cameras have, during the war, been the
ideal of all aerial photographers, but the time has been too short since
the necessary conditions have been understood for that lengthy
development work and those complete service tests which are so necessary
to develop all automatic apparatus. Several designs which are now being
perfected may be counted on to take us a long way toward this ideal.

On the other hand, that military ideal which leaves the camera operator
the greatest possible freedom for other activities, is apt to be
entirely reversed in peace. The camera operator can now be required to
be an expert, who will be free to change plates or filters and to
estimate exposures, instead of giving his best efforts to the problem of
defence. For him a simple and reliable hand-operated or semi-automatic
camera is entirely satisfactory, and the great expense of complicated
automatic apparatus has no longer its former justification.

=Camera Suspension.=—Perhaps the most pleasing prospect before the
aerial photographer as he turns from war to peace work is that of having
planes built for and dedicated primarily to photography. Instead of his
camera being relegated to an inaccessible position, picked after the
plane design has been officially “locked;” instead of yielding first
place to controls, machine gun and ammunition; instead of being
jealously criticised for the space and weight it takes up, the camera
can now claim space, weight, and location suitable for any likely aerial
photographic need. High speed no longer will be vital, and slower
planes, permitting longer exposures in inverse ratio to their speed,
will be chosen for photographic purposes.

A development which is sure to intrigue many investigators is the
gyroscopically controlled camera. This has its chief _raison d'être_ in
precision mapping, whose possibilities from the air will undoubtedly be
intensively studied at once. With the automatically leveled camera will
come renewed attention to indicators of time, altitude, and direction,
with the ultimate goal of producing aerial negatives that show upon
their face the exact printing and arranging directions necessary to put
together an accurate map.

=Sensitive Materials.=—Manufacturers of plates and films will direct
efforts toward producing emulsions of good contrast, high color
sensitiveness and high effective speed, especially when used in
conjunction with the filters necessary for haze penetration. Exposure
data will be accumulated and exposure meters appropriate for aerial work
will be developed.

=Color Photography.=—Color photography from the air by any of the
screen-plate or film-pack methods is probably out of the question
because of the long exposures required. The screen-plates are unsuitable
also because of the relatively large size of their grain compared to the
detail of the aerial photograph. Ordinary three-color photography, using
three separate negatives, is always subject on the earth's surface to
the difficulty that the three negatives must be exposed from the same
point of view, either in succession or by means of some optical
arrangement which is costly from the standpoint of light. In
photographing from the air this difficulty of securing a single point of
view for the three photographs is absent. Three matched cameras, side by
side in the fuselage, have identical points of view as far as objects on
the earth below are in question. Consequently, three-color negatives are
entirely possible, and indeed will be simple to make as soon as plates
of adequate color sensitiveness and speed are available. Probably the
new Ilford panchromatic plate has the necessary qualities.

=Night Photography.=—The searching eye of photography was so omnipresent
in the later stages of the Great War that extensive troop movements and
other preparations had to be carried out either in photographically
impossible weather or else at night. The natural reply to the
utilization of the cover of night is to “turn night into day” by proper
artificial illumination. At first thought it might well appear that the
task of illuminating a whole landscape adequately for airplane
photography is well-nigh hopeless by any artificial means. On one hand
we have the very short exposures alone permissible; on the other the
fact that the intensity of daylight illumination is overwhelmingly
greater than those common in the most extravagant forms of artificial
illumination.

Toward the close of the war, however, actual experiments made with
instantaneous flashes of several million candlepower showed that if
proper means were provided to insure the flash going off near the
ground, and if its duration were made no longer than about 1/50 second,
interpretable photographs were obtainable on the fastest plates. It
appears, therefore, merely a matter of manufacturers perfecting the
technique of flash production, and of inventors providing the launching
and igniting devices to push this kind of photography to the practical
stage. The achievement of night photography cannot fail to have an
enormous effect on future tactics.

The technique of night photography may take either of two directions. On
one hand we may develop flashes of the requisite intensity to give all
their light in 1/100 second; on the other hand, it may prove more
feasible to use flashes of longer duration and to arrange for the camera
shutter (of the between-the-lens type) to be exposed synchronously with
the middle of the flash. One way, frequently suggested, to use these
longer flashes would be to trail the charge on a long wire, through
which the ignition is effected electrically. This is not likely to be
satisfactory, however, for the resistance of a wire is so great that
when the plane flies at any practical height, the trailed flash, if it
reaches near the ground, will be forced to a very great distance behind.
Probably the best solution will involve accurate synchronizing of the
fuse in the freely dropped sack of flash powder with the exposing
mechanism in the camera.




                             CHAPTER XXXII
                      PICTORIAL AND TECHNICAL USES


Aside from their element of novelty, aerial photographs have undoubted
qualities of beauty and utility. The “bird's-eye view” has always been a
favorite for revealing to the best advantage the entire form and
location of buildings and of other large objects. Heretofore such views
have usually had to be drawn by an imaginative artist.

[Illustration:

  FIG. 188.—Rheims Cathedral.]

Aerial oblique views possess the virtues both of pictures and of plans.
They are destined to be extensively used in the study of architecture
(Fig. 188). Cathedrals, castles, town halls, particularly those still in
their cramped medieval surroundings where they can never be seen in
their entirety from the ground, come forth in all their beautiful or
quaint proportions from the airman's point of vantage. Stereoscopic
aerial views are destined to occupy a valuable position also. Stereo
prints of the famous buildings of Europe, taken from the air, will give
to the prospective traveler or the arm-chair tourist a many fold more
accurate idea of their construction than will any number of mere surface
views.

[Illustration:

  FIG. 189.—A portion of Vienna seen from the air, during a “propaganda
    raid.”

  Italian official photograph.]

[Illustration:

  FIG. 190.—The Rialto bridge, Venice.

  Italian Air Service photograph.]

[Illustration:

  FIG. 191.—A partly developed suburb.]

A vertical aerial photograph is most closely akin to a map, but has
advantages over any ordinary surveyor's product. As a guide it is
infinitely superior to the best draftsman's diagram, for it provides a
wealth of detail whereby the traveller may definitely locate himself. At
a single glance he notes the objects of interest within his radius of
easy travel. The guide-book of the future will therefore be incomplete
without numerous aerial views, both vertical and oblique. As an
illustration of the peculiar merit of the view from the air, consider
the photograph of Vienna made during d'Annunzio's “propaganda”
bombardment. Or the picture of the Rialto bridge (Fig. 190). No ordinary
photograph from land or water suggests the central roadway and no map
shows the beauty of its elevation. Both are shown here, as well as an
intimate view of the arched and pillared courtyard of the Fondaco de'
Tedeschi to the right.

[Illustration:

  FIG. 192.—A sea-side resort.]

[Illustration:

  FIG. 193.—A bathing beach seen from the air.]

Airplane photographs will undoubtedly be widely used in certain fields
of advertising. Architects and real estate agents may be expected to
display their wares by the aid of aerial views. A well-planned country
estate or golf course, or a suburban development (Fig. 191), can be
shown with a completeness, both as to environment and stage of progress
which no other form of representation can approach. A sea-side resort
can now show the extent and grouping of its natural and artificial
amusement features in a single picture (Fig. 192). Even the extent of
its bathing beach under water is revealed to the aerial photographer
(Fig. 193). Real estate agents can utilize aerial photographic maps of
cities to great advantage. On these their properties can be pointed out,
with the nature of their surroundings shown at a glance, together with
their relation to transportation, schools, churches, shopping districts,
parks, or factories. The future purchaser of lots in a distant boom town
will no longer be satisfied with a map outlining the streets with
high-sounding names. He will demand an authentic aerial photograph,
showing the actual number of houses under construction, the streets,
gutters and sidewalks already laid, the size and planting of trees.

[Illustration:

  FIG. 194.—Mt. Vernon from the air.]

[Illustration:

  FIG. 195.—A contrast in roofs. The Capitol retains its individuality,
    while the White House loses all character when seen from above.]

The study of landscape gardening is another field for which the aerial
photograph is peculiarly fitted. A collection of oblique pictures of the
châteaux and palaces of Europe showing their approaches and grounds, or
of the historic estates of our own South, (Fig. 194), will be worth more
to the prospective designer of a country estate than maps and ground
pictures can ever be. Closely allied to landscape gardening is city
planning, for which the aerial map will be quite indispensable. The
appearance of a city from the air may indeed become a matter of pride to
its inhabitants, and not only the arrangement of streets and parks, but
even the character of the roofs of the buildings, be the subject of
study (Fig. 195).

[Illustration:

  FIG. 196.—An aviation field under construction; early stage.]

Engineers and constructors will depend more and more on preliminary
photographic surveys as a basis for locating their operations. At the
later stages of their work they will use aerial photographs for
recording progress. Periodic photographs of buildings in process of
construction, such as are now made from the ground, are much more
illustrative when made from the air. Only from above is it possible to
obtain in a single picture the progress of the complete project, such as
the construction of an aviation field (Figs. 196 and 197) or of a
shipyard. The building of large structures—bridges, hotels, ships on the
stocks—particularly demands aerial views if the foreground is not to
eclipse the center of real interest.

[Illustration:

  FIG. 197.—An aviation field under construction; later stage.]

News events will soon call for an aerial photographic service. Already
we are seeing newspapers and magazines featuring aerial photographs of
the entry into conquered cities and the parades of returning fleets.
Accidents, fires, floods and wrecks, of either local or national
interest, can best be represented by this newest form of photography.

[Illustration:

  FIG. 198.—The crater of Vesuvius.

  Photograph by Royal Italian Air Service.]

The photographing of wrecks, fires and floods suggests the importance of
aerial views to insurance underwriters, who require the most minute
information on the characteristics of buildings in every neighborhood,
and on the extent and nature of damage done. Marine insurance companies
might with profit use the airplane camera to help estimate the chances
of salvage of a stranded ship or a vessel foundered in shallow waters
(Fig. 181).

[Illustration:

  FIG. 199.—Waves set up by a ship—of interest to the naval architect.]

Numerous scientific uses for aerial views seem likely. Prominent among
these is their use in geology, for the study of the various forms of
earth sculpture. Pictures from the air of extinct volcanoes will give
information as to their configuration that would otherwise require
months of painstaking survey to obtain. Aerial photographs of active
volcanoes (Fig. 198), showing the results of a succession of
outbursts—one obliterating the other—would prove of the greatest value,
especially when studied in conjunction with other scientific data, the
whole making a record unobtainable by any other means.

In earthquake regions—notably Southern Italy and Japan—the changing
coast lines, shallows and safe harbors, could be promptly ascertained
after the subsidence of each fresh shock, with a consequent keeping open
of trade routes and often the saving of life. River courses, glacier
formations, cañons, and all the larger natural formations which man
usually sees only in minute sections, and which he must build up in his
mind's eye or by models, are today quickly and accurately recorded by
the camera in the air. Such formations as coral reefs, whose
configurations can now be accurately learned only by laborious surveys
of a limited number, could be studied in quantity and with heretofore
unknown satisfaction as the result of a single expedition with a
ship-carrying seaplane and aerial camera.

Another scientific field—probably one of many similar ones—lies in the
study of the waves set up by ships (Fig. 199). These are of extreme
importance in the realm of naval architecture, but before the day of the
airplane could never be easily studied in full scale.




                             CHAPTER XXXIII
                        EXPLORATION AND MAPPING


Aerial photographic mapping in war-time has been almost entirely
confined to inserting new details in old maps. For such work some
distortion or a lack of complete information on altitude and directions
is not a serious matter, because the known permanent outlines serve as a
basis. Furthermore, in so far as outline maps are concerned, as
distinguished from pictorial maps, these have been drawn on the ordinary
scales, and with the ordinary conventions of engineering map practice.

Aerial photography may be used in the future in practically the same
way, as an aid to the quick recording of those minute details which
would ordinarily consume an enormous amount of labor to survey directly.
The region shown in Fig. 200 affords a good illustration. A discouraging
amount of time and effort would be required to map this section of
Virginia by the usual methods, while the smallest curve of creek and
shore is instantly and completely recorded on a single photographic
plate. But there are other possibilities, diverging from this
application both toward greater and lesser requirements for precision.

Pictorial maps, in which the actual photographs figure, promise to be an
essential part of the airman's equipment, whether he be pilot or
passenger, mail carrier or sportsman. Without any pretention to detailed
accuracy of location, these maps will show, in strip or mosaic form, the
general appearance of the country to be traversed, with particular
reference to good landing fields and other points of interest to the
aviator. Vertical pictorial maps may be supplemented by obliques giving
the view ahead, whereby the pilot may direct his ship. Thus the
Washington monument as seen by the pilot from Baltimore is a truer guide
than is the country beneath him. The crossing of mountain ranges is
another case where the oblique picture will be more useful than the
vertical (Fig. 201).

[Illustration:

  FIG. 200.—An aerial photographic survey of ground difficult to cover
    by ordinary surveying methods.]

[Illustration:

  FIG. 201.—Seeking out mountain passes.]

Contrasted with the merely pictorial maps will be precision surveys.
Whether it will prove practical to make these entirely from the air is
still an open question. It is to be assumed that cameras can be
constructed with lenses having negligible distortion of field, with
between-the-lens shutters to obviate the distortions due to the
focal-plane type, with auxiliary devices for indicating compass
direction, altitude, and inclination, or with gyroscopic mounting so
that an inclination indicator is unnecessary. The application of aerial
photography to precision mapping will depend upon the perfection which
such cameras attain, as estimated by the permissible errors in this form
of mapping. Entire dependence on photography, as in uncharted regions,
is likely to be worked up to slowly, beginning with a stage of rather
complete triangulation of natural or artificial points—say three in each
constituent picture—then through several stages each successively
employing fewer and fewer well determined points. The photographic
mapping of some of our Western States will be greatly facilitated by the
100-yard squares into which the land is divided and already marked in a
manner which shows clearly in aerial photographs.

A theoretical possibility is the plotting of contours from stereo-aerial
pictures. Given two elements of a stereoscopic pair, taken from points
whose separation is known, the position of any point in space shown in
the stereoscopic view can be determined by the use of the
stereo-comparator. This is an instrument already employed in mountain
photo-surveying, which consists essentially of a compound stereoscope in
whose eye-pieces are two points movable at will so that the relief image
formed by their fusion can be made to coincide with any chosen part of
the landscape. The chief difficulty in the application of this idea to
aerial work is to fix the base line. This problem may be met in some
cases by using stereo obliques, and getting the base line by
simultaneously made vertical photographs of well surveyed territory
beneath. Possibly also methods can be developed by which photographs
from two or more known altitudes may furnish the requisite data.

[Illustration:

  FIG. 202.—Business section of Hampton, Virginia. A survey made by a
    single instantaneous exposure.]

[Illustration:

  FIG. 203.—Mosaic map of the City of Washington. White rectangle shows
    portion included in next figure.]

[Illustration:

  FIG. 204.—Portion of Washington mosaic, full size.]

City mapping is a field for which aerial photography is peculiarly
fitted (Fig. 202). A complete map of a large city is a labor of years.
In fact, a modern city is always dangerously near to growing faster than
its maps. An aerial map, on the contrary, can be produced in a few
hours. Paris was mapped with 800 plates in less than a day's actual
flying. Washington was completely mapped in 2½ hours, with less than 200
exposures. The entire map is shown, on a greatly reduced scale, in Fig.
203, while Fig. 204 shows a small portion of it in full size, from which
can be obtained an idea of the dimensions of the original. These maps,
while not accurate enough for the recording of deeds and mortgages, yet
serve the majority of needs. There is indeed no reason why with long
focus cameras, given several accurately marked points, the photographic
map of a piece of real estate should not be made with all the accuracy
needed, still leaving the whole process of partial surveying helped out
by photography an enormously simpler one than the usual method.

Rougher types of surveying, in open country, offer a most promising
opportunity. Railway surveys, showing the character of the country:
passes through mountain ranges: the available timber and other materials
of construction. Canal routes, with the available sources of water
supply, and the best choice of course to avoid deep cuttings and
aqueducts. Irrigation projects, with the natural lakes, river courses
and valleys, which may be dammed to form storage basins. Coast, river
and harbor surveys are possible by aerial means with a promptness and
frequency which should entirely revolutionize the making of maps of
waterways. Shifts in channels and shallows, even of considerable depth,
stand out prominently in the aerial photograph. The actual bottom, if
not more than three or four meters down—as in a bathing beach—shows in
the aerial photograph (Fig. 193), while the varying surface tints caused
by light reflected from the bottom at far greater depths are readily
differentiated by the camera from the air. An instantaneous photograph
will thus perform the work now done by a week's soundings. Fig. 205,
taken near Langley Field, shows how the aerial photograph may be used to
chart natural channels, while Fig. 206 shows the dredged channels of the
port of Venice. Navigation of such a river as the Mississippi with its
shifting bars may come to be guided by monthly or even weekly aerial
photo maps.

[Illustration:

  FIG. 205.—Shallows and channels revealed by the aerial photograph.]

[Illustration:

  FIG. 206.—Venice from the air, showing dredged channels.

  Italian official photograph.]

[Illustration:

  FIG. 207.—Bengasi, a North African town, surveyed for the first time
    from the air.

  Italian official photograph.]

[Illustration:

  FIG. 208.—Thurnberg on the Rhine.

  Photograph by photographic section A. E. F.]

Among other uses for aerial photography will be the location of timber.
As one illustration, may be taken the discovery of mahogany trees. Their
foliage at certain times of the year is of characteristic color. This
may be recorded on color sensitive plates with a scientifically chosen
filter, and the cutting expedition sent out with the photograph as a
guide. In this as in other cases where rough or unexplored country is to
be covered, it is a question whether the airplane will after all be the
most feasible craft, on account of its necessarily rapid rate of travel,
and its need for known landing fields. The dirigible of large cruising
radius, which can seek its landing field at leisure, is probably
indicated for this kind of work. It may indeed, as already hinted, prove
to be the chief photographic aircraft of the future.

Archæological surveys offer a fascinating opportunity for airplane or
dirigible balloon photography to render scientific service. Buried in
desert sands or overgrown with tropical vegetation the ancient cities of
Asia Minor, of Burma, and of Yucatan evade discovery, and even when
found remain unmapped for decades. Discovery and mapping can now go
hand-in-hand. The topography of barbaric or colonial towns and villages,
whose importance could not warrant elaborate surveys, but which should
nevertheless be a matter of record, will be quickly and easily plotted
by photography (Fig. 207). To this day who knows how the streets run in
Timbuctu, and how, save from the air, can we ever map the teeming cities
of China? He who would follow in the footsteps of Haroun-al-Raschid can
even now explore the by-ways of Bagdad by the aid of the Royal Air Force
photographic map!




                                 INDEX


 Aberrations, lens, 47, 54

 Aberration, spherical, 47, 54
   chromatic, 48, 54

 Acetylene light, 284

 Advertising, use of aerial photography in, 392

 Aero 1 and Aero 2 filters, 241

 Airplane, as camera platform, =20=
   types of, 24

 Air speed, 27, 308
   indicator, 33, 34

 Alcohol, use of in plate drying, 275
   use of in print drying, 286

 Altimeter, 33, 174
   reading recorded on film, 135, 171

 Altitudes of flight for photography, 61, 224

 Anastigmatic type of lens, 47

 Aperture, lens, 39, 44, 56, 57

 Archæological surveying, 413

 Astigmatism, 49, 50, 54

 Auxiliaries, camera, =163=
   installation of, 214


 Bagley camera, 286, 314

 Balance, of camera, 96
   of plane, 157, 208, 217

 Balloons, 15, 16, 18

 Banking, 27, 324

 Batteries, storage, 150, 156

 Bay, camera, 120, 210

 Bellows, camera, 65, 95

 Biplane, 24

 Blast marks in front of guns, 363

 Bleaching out print to leave interpretation marks, 367

 Boat, flying, 25, 374

 Bowden wire, 103, 106, 108, 111, 114, 116, 118, 126, 129, 136, =163=

 Brightness, range of, 221, 225

 Burchell photographic slide rule, 303, 339


 Camera, airplane, =39=, 384
   automatic, 18, 43, 90, 116, =124=, 125, 311
   B. M., =120=, 203
   Bagley, 286, 314
   Brock automatic plate, =126=
   C type, 43, 87, 103, =109=
   classification of, 43
   deMaria, 86, 89, 103
   deRam, 82, 93, =121=, 129, 157, 205, 214, 326
   E type, 43, 87, 103, =109=
   elements of, =42=
   film, _see_ Film cameras
   Folmer automatic plate, =126=
   hand held, =95=, 321, 369
     English, =99=
     German, =99=
     U. S. Air Service, =100=
   Ica, =103=
   Lamperti (Italian), =112=, =211=
   L. B., =120=
   long focus, 103, 301, 324
   L type, 43, 82, 94, 102, =117=, 162, 210
   M., =106=, 203
   non-automatic, 43, =102=
   Piserini and Mondini, =111=
   semi-automatic, 43, =116=, 149
   stereoscopic, 341

 Camouflage, filters for the detection of, 225, =243=
   stereoscopic views and, 329, 358

 Ceiling of plane, 27, 130

 Channels, detection of by photography, 408

 Chemicals, photographic, =257=

 Chlorhydrochinon developer, 261

 Chromatism, lateral, 49, 54

 City planning, use of aerial photography in, 396

 Clock-work for driving cameras, 149, 155

 Clouds, 224, 242

 Collimator, 66

 Color, coefficient or index of negative, 259
   filters, _see_ filters
   photography, =385=
   sensitive plates, 15, 174, 233, 237
   sensitiveness of film, 131
   sensitizing, methods of, 235

 Coma, =47=, 48

 Communication, means of on plane, 296

 Compass, 33, 173
   reading recorded on film, 135, 171

 Cone, camera lens, 42, 114
   interchangeable, 108, 120

 Construction operations, aerial photographic records of, 396

 Contact, imperfect in printing, 279, 284 prints, 45, =279=

 Contacts, electric, on plane, 163

 Contours by stereo aerial photography, 404

 Control, distance, of camera, 110, 163
   speed, of camera, 136, 144, 157

 Controls, duplicate, 19, 25, 195, 209, 214
   of plane, 21, 26

 Convergence point, 337

 Cord for adjusting shutter aperture, 82

 Core rack development, 271

 Contrast, in brightness on earth's surface, 221
   in photographic emulsions, 15, 230, 236, 258

 Counter, exposure, on magazine, 88
   on release, 164

 Covering power of lens, 44, =49=, 50, 58

 Crabbing, 27, 308

 Cradles, camera, 195

 Cross wires, 23
   insertion of camera through, 42, 210

 Curtain, auxiliary shutter, 80, 84, 106
   speed of travel of shutter, 74
   uniformity of, 76, 82, 84, 86, 315

 Cylinders, relation between vibration and number of, 185


 Dark slides, double, 87, 99

 Daylight, intensity of, 222

 Definition, lens, 44

 Density, of air, effect of on propeller, 154
   of photographic image, 228

 Developers, for plates and films, =257=, 260
   for papers, =262=

 Developing machines, film, 133, =273=
   Ansco, 273
   Brock, 274
   Eastman, 274
   G.E.M., 273

 Development, core rack method of, 271
   factor, 228
   film, =272=
   methods of, =267=, 269
   of prints, =286=
   speed of, 236, 267
   tank, 270
   time, 269

 DH-4 plane, 210, 217, 296
   photographic, 213

 DH-9 plane, 296

 Diaframs, to equalize illumination of plate, 78
   lens, 48, 58

 Dilution coefficient of developer, 258

 Dirigibles, 15, 16, 413

 Distortion, absent with between-the-lens shutter, 74
   barrel, 51
   due to camera tilting, 206, 286, 305, 315
   in aerial maps, 317
   lens, 39, 44, 51, 54, 56, 62
   pin-cushion, 51
   produced by film shrinkage, 237
   produced by focal plane shutter, =74=
   produced by glass plate in front of film, 131
   with wide angle lenses, 63
   “Dodging” in printing, 279, 315
   Doors in plane for camera to work through, 214

 Drying of films, =267=, 276
   of plates, 267, 275
   of prints, =286=


 Earth, appearance of from plane, 30

 Eastman apron film developing machine, 274
   twin reel film developing machine, 274

 Efficiency, propeller, 155
   shutter, 70, =72=, 76

 English aerial photographic practice, 45, 46, 283, 291, 340

 EK filters, 241

 Electric, drive for cameras, 116, 119, 123, 145, 149
   generator operated by motor, 146
   motor, characteristics of, 151, 156
   motor, service, 163

 Elevation possible to detect in stereoscopic views, =340=

 Emulsions, photographic, characteristics of, =227=

 Enlarging, 45, =279=, 283
   camera, 283

 Enlarging versus contact printing, 59

 Exhaustion of developer, 259

 Exploration, use of aerial photography in, 401

 Exposure, data charts, 250
   distance between for mosaic maps, 307
   distance between for stereos, 336
   estimation of, 248
   limitations to, 247
   meters, 251
   meter, Wynne, 251
   of aerial negatives, =247=
   relation between motion of plane and, 68, 185
   under, period, 230


 Field, angular, of lens, 57, 302
   flatness of lens, 56

 Field laboratory, mobile, 268

 Film cameras, 43, =130=, 134
   Brock, 138
   Duchatellier, 131, 136
   F type, 134, 171
   G.E.M., 138
   German, 64, 76, 139, 317
   K type, 142, 203, 214

 Film, celluloid, 130
   backed, 133
   changing in the air, 137, 144
   color sensitiveness of, 237
   cut, 275, 278
   development of, 130, 272
   drying, 131, 276
   means for holding flat, 130, 131
   relative performance of compared to plates, 237
   satisfactory kinds for aerial work, 238
   shrinkage, 237

 Filters, 15, 58, 106, 174, 224, 233, =239=, 241, 243
   effects secured by use of, 241

 Filters, gelatin, 244
   glass, 245
   holders for, 67, 106, 224
   ratio, 240

 Fixing bath, 262

 Flaps, auxiliary to shutter, 80, 84, 85, 98, 99, 101, 119
   to protect lens, 214

 Flash lights, 18, 386

 Flying boat, 374

 Focal length, relationship of lens characteristics to, 55, 57
   requirements for, 39, 42, 58, 61, 103

 Focal plane, 50
   shutter, 70, 71
   distortion by, 74
   performance of, 86
   types and representative, 80

 Focus, depth of, 44
   effect of temperature on, 41, 65
   fixed, in aerial cameras, 40
   ground and air, 65

 Focussing, automatic, 284
   by parallax, 65, 66

 Fog, atmospheric, 224
   photographic, 237, 258

 French aerial photographic practice, 45, 46, 283

 Friction disc speed control, 136, 159, 160

 Fuselage, 21, 87
   shape and size depending on type of engine, 23

 Future of aerial photography, =383=


 German aerial photographic practice, 63, 103, 209

 Glass, optical, used in lenses, 44

 Gloves, handling apparatus through, 41, 89

 Governor for camera speed, 159

 Graphite, use of to prevent static discharge, 134

 Gravity, action of in aerial cameras, 41, 112, 115, 119
   center of, should not change in cameras, 125, 207
   change of center of, in magazines, 92, 112
   depended on in magazines, 88
   handles best at center of, 96
   pseudo, in moving vehicle, 188
   support at center of, 182, 203

 Ground speed, 27, 307
   indicator, 311

 Guide-books, illustration of, 15, 392

 Guides for inserting magazines, 41

 Gyroscope, 189

 Gyroscopically controlled instruments, 29, 174, 192, 312

 Gyroscopic erector, 188
   mounting of camera, 187
   stabilizer, Gray, 190


 Hardener, acid, 262

 Hand operation of deRam camera, 121, 125, 129

 Haze, 15, 30, 223, 233, 239

 Head resistance and weight, equivalent, 156

 Heat, effect of on plate sensitiveness, 175, 232

 Heater, electric, in camera, 142, 174, 232

 Horizon, photography of to indicate
   inclination, 174
   position of, 30

 Hurter and Driffield sensitometric curve, 227


 Illumination of field by lens, 50, 54, 56, 57

 Image of point source, size of, 49, 54, 56
   size of in relation to focal length, 59

 Incidence, angle of assumed by plane at high altitudes, 206

 Inclinometer, 33, 35, 171, 173

 Indicators, distance, 163, 295

 Inertia, 229, 257

 Installation, camera, 24, 208

 Instructions, operating, to be placed on apparatus, 42

 Instruments, airplane, 30

 Instrument board, 30
   photographing, 170

 Intensification of aerial negatives, 262

 Intensity of daylight, 222

 Interpretation of aerial photographs, 17, 40, 351

 Interval between exposures, 40, 158, 305, 307
   for stereoscopic pictures, 334, 338
   methods of regulating, 124

 Isochromatic plates, 233

 Italian photographic practice, 122, 377


 Jamming of cameras in operation, 119, 120


 K_{1} and K_{2} filters, 239

 Keeping power of developer, 259

 Kites, 15, 16, 18


 Laboratory, mobile photographic, 269

 Landscape gardening, use of aerial photography in, 395

 Latitude of plates, 229

 Lens, 42, 39, =44=, 383
   aperture, 39
   characteristics, 46
   mounts, 65
   suitable for aerial photography, 62
   symmetrical, 52
   telephoto, 61
   testing and tolerances, 52
   unsymmetrical, 52
   wide angle, 62

 Levels, spirit, on camera, 95

 Light, distribution of in aerial view, 221
   trail method of testing camera mountings, 183

 Loop, centrifugal force in, 29


 Machine gun ring as camera mount, 321

 Magazines, =87=
   bag, 88, 101
   Bellieni, 92
   Chassel, 92
   deMaria, 89, 98
   Ernemann, 89
   Folmer, 90
   Fournieux, 92
   Jacquelin, 92
   Piserini and Mondini, 90
   Ruttan, 92

 Magazine racks, 94
   installation of, 217

 Mapping, 64, 135, 185, 186, =304=, 401
   precision, 317, 404

 Maps, mosaic, 17, 39, 64, 314
   sketch, 314

 Marking of negatives, 278

 Metol-hydrochinon developer, 261

 Mirrors for oblique photography, 324, 326

 Monoplane, 24

 Motions of camera, 179

 Motive power for aerial cameras, =145=

 Mounting, camera, 102, =103=, 179, 183 384
   bell-crank, 120, 198, 203
   Brock, 139, 207
   center of gravity, 205
   floor, 195
   G. E. M., 138, 207
   Italian, 207
   outboard, 194
   parallel motion, 198
   pendular, 185
   tennis ball, 196

 Mounting, camera, turret, 312
   of prints, 316
   of stereograms, 346

 Movement, of film during exposure, 75, 142
   of image, permissible, 68

 Moving pictures from plane, 350

 Mud splashing on camera, 214


 Naval aerial photography, =368=

 Negative lens sight, =168=, 299, 305

 Night photography, =386=

 Numbering devices in cameras, 169


 Oblique views, 39, =320=
   angles at which taken, 40, 321
   exposures for, 69
   filters for, 242
   use of hand cameras for, 95

 Observer, function of in aerial photography, 291, 295, 304

 Oil spray from motor, 214

 Opacity, 228

 Opening for camera, 211

 Opposite directions, shutter to move alternately in, 76, 139

 Orthochromatic plates, 233

 Overlaps, for mapping, 307
   for stereoscopic views, 40
   on a turn, 64, 139


 Panchromatic emulsions, 233
   plates, 49, 238

 Panoramic views, 321

 Parallax method of focussing, 65, 66

 Parallel, flying in, 308

 Photographic planes, special, 213

 Pilot, function of, in aerial photography, 291, 295

 Pinpoints, 39, 291

 Pistol grip for hand cameras, 95, 96

 Plate holders, 87

 Plates, bathed, 235
   behavior of compared to film, 237
   color sensitive, 233
   iso- and ortho-chromatic, 233
   panchromatic, 233
   satisfactory kinds for aerial work, 238
   self screening, 245
   shape of, 63
   size of, 43, 62

 Plumb line, behavior in banking plane, 28

 Ply-wood veneer construction, 23, 211

 Positype paper, 238

 Potassium carbonate for drying plates, 276

 Power required to drive cameras, 125

 Pressure, of shutter curtain, 131
   plate, for holding film flat, 131, 141

 Printing, =279=
   contact, 279
   machines, 279
   media, 252

 Prints, paper, 253
   development of, =286=

 Prisms for oblique photography, 324, 326

 Propeller, characteristics, 152
   constant speed, 129, 159
   drive for cameras, 102, 116, 119, 135, 136, 144, 157, 158
   position of, 120
   variable speed, 159

 Pump, for producing suction on film, 132

 Punch marks on film, 144, 272

 Pyro developer, 261


 Racks, negative, 271

 Real estate, aerial mapping of, 393

 Rectifying, 286, 305
   camera, 314

 Reduction of aerial negatives, 262

 Release, shutter, 96, 99
   duplicate, for pilot, 164

 Release, shutter, time controlled, 124

 Relief, criterion for correct, 335, 344
   exaggerated, 337
   impression of, produced by motion, 349

 Resolving power of plates, 59, =235=, 260

 Richard stereo printing frame, 283, 348

 Rinsing of plates, 259

 Rubber, sponge, use of in camera mountings, 195, 203


 Safe lights, photographic, 269

 Safety catch on camera mounting, 203
   device on camera driving mechanism, 162

 Salvaging of ships, aerial photography and, 399

 Seaplane, 25, 374

 Self screening plates, 245

 Semaphore signalling in plane, 297

 Semperfocal enlarging camera, 284

 Sensitized materials, requirements for, 225

 Sensitometry, =227=
   of papers, 253

 Shadows, compass directions from, 356
   proper direction for, in examining prints, 352

 Shaft, flexible, 119, 123, 142, 161

 Sheaths, plate, 87, 88, 93, 126, 170

 Shrinkage, film, 237, 317
   paper, 285, 315, 317

 Shutter, 42, 68
   between-the-lens, 58, 70, 112, 115, 316, 387
   efficiency, 70, =72=, 76
   focal plane, 70, 71, 73
   focal plane, double for stereo work, 341, 345
   focal plane, moving alternately in
   opposite directions, 76
   focal plane, types of, 80

 Shutter, Folmer, 80
   Ica, 81
   Klopcic, 78, 84, 98
   release, 96, 99, 124
   speed, 39, 40, 58, 70, 249
   testing, 76
   testing apparatus, 77

 Sights, =164=, 166, 296, 301, 327
   adjustable for angle of incidence of plane, 169
   attached to plane, 167
   negative lens, 168, 299, 305
   rectangular, 98, 167, 373
   stereoscopic, 338
   to indicate size of field, 166, 373
   tube, 101, 166

 Single-seaters, carrying cameras in, 114, 211

 Slide rule, photographic, 303, 339

 Solenoid, 151, 163

 Sound, not to be used for indication in plane, 164

 Spacing of pictures in film camera, 144

 Speaking tubes on plane, 296

 Speed, of development, 236, 267
   of plates, 228, 236
   criteria of, 230
   effect of temperature on, 175, 232
   variable, control of camera, 144, 151

 Spotting, 64, 125, =291=

 Spring motors, 149, 155

 Springs, use of in aerial cameras, 41
   in magazines, 88

 Stabilized camera, 95, 187

 Static electric charges on film, 131, 133

 Stereo-comparator, 404

 Stereo-oblique views, 115, 321, 343

 Stereoscopes, 331

 Stereo printing, 283

 Stereoscopic cameras, 341, 344
   effect, absence of at flying heights, 30, 334

 Stereoscopic photography, 329
   mounting, 346
   pictures, fusion of without instruments, 330, 343
   sights, 338
   views, 39, 40, 64
     uses for, 348
   vision, principles of, 329

 Stop-watch attached to shutter release, 310

 Strap, on hand camera, to go around observer's neck, 99
   on plate magazine, 87
   to go over hand, on hand camera, 100

 Stream lines, 26
   lined hood, 214

 Suction for holding film flat, 131, 132, 142
   advantages of continuous and intermittent, 132

 Surveying by aerial photography, 401


 Tank development, 270

 Tearing edges of prints, 317

 Telephones on planes, 296

 Telephoto lens, 61

 Temperature, coefficient of development, 258
   effect of on focus, 41
   effect of on mechanical functioning, 41, 102, 125
   effect of on plate speed, 232
   limits of development, 258

 Test chart, lens, 52

 Threshold value, 230

 Thrust, propeller, 152

 Timber, location of by aerial photography, 413

 Tone rendering, correct, 226, 230, 239

 Touch, sense of, not dependable in plane, 41, 125, 164

 Trailer, photographic truck and, 268

 Transmission of power to camera, 161

 Transparencies, 252, 330

 Transparency, 227

 Trays, camera, 195

 Tri-color ratio, 235

 Triplane, 24

 Tuning fork, used in shutter tester, 79

 Turbine, wind, for driving camera, 116, 127, 144, 147, 158


 Uniformity of curtain speed in focal plane shutter, 76, 82, 84, 86, 315

 Unit system of camera construction, 42

 Uprights, camera, in plane, 209

 Uses for aerial photography, =388=


 Velocity constant, 258

 Veneer construction, 23, 209, 211

 Venturi tube, 132, 142, 144

 Vibration, 16, 18, 26, 40, 41, 58, 102, 179

 Volcanoes, photography of, 15, 399


 Water for mixing chemicals, 262

 Watkins factor, 258

 Weight and head resistance, equivalence, 156
   of film compared to plates, 101, 130
   of deRam camera, 123
   of hand cameras, 96
   of K type camera, 144
   of K film roll, 144
   of L type camera, 117
   of M type camera, 109
   of storage batteries, 157

 Wind, flying against, 68, 308
   motor, 146

 Windows in side of plane, 214, 328

 Wire, barbed, appearance of in aerial photograph, 360




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Transcriber's note:

 1. Silently corrected typographical errors.

 2. Retained anachronistic and non-standard spellings as printed.