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OPTICAL PROJECTION

A TREATISE ON THE USE OF THE LANTERN IN
EXHIBITION AND SCIENTIFIC DEMONSTRATION

BY

LEWIS WRIGHT

AUTHOR OF 'LIGHT: A COURSE OF EXPERIMENTAL OPTICS'

5TH EDITION

RE-WRITTEN AND BROUGHT UP-TO-DATE BY

RUSSELL S. WRIGHT, M.I.E.E.

IN TWO PARTS

PART I

_THE PROJECTION OF LANTERN SLIDES_



_WITH ILLUSTRATIONS_



LONGMANS, GREEN, AND CO.
39 PATERNOSTER ROW, LONDON, E.C. FOURTH AVENUE & 30TH STREET, NEW YORK
BOMBAY, CALCUTTA, AND MADRAS

1920

(_All rights reserved_)

       *       *       *       *       *


{v}

PREFACE TO THE FIFTH EDITION

The first edition of this work was written by my father, the late Mr. Lewis
Wright, and was published in 1890.

The reception that it received testified to the fact that it met a
long-felt want, and successive editions were published in 1895, 1901, and
1906.

My father, unfortunately, met his death in a railway accident in 1905, and
the corrections and additions to the last edition, which had been to a
certain extent prepared by him, were completed and written by myself, and
the work as published then was again reprinted in 1911.

As the original text is now thirty years old, it has seemed better entirely
to re-write the whole book rather than make fresh revisions, the more so as
the last ten years have seen great advances in the science of Lantern
Projection, and especially in the developments of Acetylene and Electric
Lighting.

It has also seemed best at the present juncture to issue the book in two
parts, the first dealing with the Projection of Lantern Slides only, and
the second with the Demonstration of Opaque and Microscopic Objects,
Scientific Phenomena and accessory apparatus, including Cinematograph
Projection.

It must of necessity be many months before this second volume can be
produced, for the simple reason that Optical {vi} Instrument Makers have as
yet hardly had time to turn round after the war and produce their new
models, and therefore any such book written now could do little more than
describe apparatus that was on the market prior to 1914.

The present work, therefore, deals solely with the exhibition of Lantern
Slides in the Optical Lantern, and as such I trust will be found of value
to Schoolmasters, Social Workers, Lecturers, and, in fact, to all who use
the lantern as a means of illustration.

  RUSSELL S. WRIGHT.
  _January 1920._

       *       *       *       *       *


{vii}

CONTENTS

  CHAPTER                                                     PAGE

  I. INTRODUCTORY                                                1

  II. THE ILLUMINANT                                             3

  III. PARAFFIN-OIL LAMPS, INCANDESCENT GAS AND SPIRIT BURNERS   6

  IV. THE ACETYLENE LIGHT                                       11

  V. LIMELIGHT AND THE ACETYLENE BLAST                          16

  VI. THE ELECTRIC LIGHT                                        39

  VII. THE OPTICAL SYSTEM OF A LANTERN                          57

  VIII. THE BODY OF THE LANTERN                                 70

  IX. LANTERN BOXES, STANDS, READING LAMPS, ETC.                76

  X. SCREENS AND SCREEN STANDS                                  79

  XI. THE PRACTICAL MANIPULATION OF A LANTERN                   82

{viii}

ILLUSTRATIONS

  FIG.                                                     PAGE

   1. Oil Lamp                                                6
   2. Inverted  Incandescent Lamp                             8
   3. Methylated Spirit Burner                                9
   4. Luna Lamp                                              10
   5. The Moss Generator                                     12
   6. The A.L. or 'Popular' Model                            14
   7. Acetylene Jet                                          15
   8. Oxygen Cylinder in hemp cover                          17
   9. Double Lever Key                                       18
  10. Fine Adjustment Valve                                  19
  11. Construction of Beard's Regulator                      20
  12. Beard's Regulator                                      21
  13. Regulator and Gauge                                    22
  14. Gas-bags                                               24
  15. 'Blow-through' Nozzles                                 25
  16. 'Blow-through' Jet                                     25
  17. Mixed Jet                                              27
  18. Mixed Jet, Gwyer pattern                               27
  19. Mixing Chamber of Jet                                  28
  20. 'Injector' Jet                                         30
  21. 'Gridiron' Saturator                                   32
  22. 'Pendant' Saturator                                    33
  23. Fallot Air Blast                                       37
  24. Fallot Air Blast, and Cylinder                         37
  25. Lime-tongs                                             39
  26. Universal Hand-fed Arc Lamp                            45
  27.                                                        46
  28. Resistance                                             49
  29. 'Scissors' Arc Lamp                                    51
  30. 'Right-angled' Arc Lamp                                52
  31. 'Westminster' Arc Lamp                                 53
  32. Arc Lamp with Induction Ring                           56
  33. The Optical System of a Lantern            _facing p._ 57
  33A. Optical System of Lantern                             57
  34. Optical System without Condenser                       59
  35. Action of Condenser                                    59
  36. Forms of Condensers                                    60
  37. Double Sliding Carrier                                 62
  38. Beard's Dissolving Carrier                             63
  39. Focussing Action of Lens                               64
  40. Achromatic Lens                                        65
  41. Petzval Combination                                    66
  42. Hughes' Short-Range Lantern                            71
  43. Long-Range Lantern                                     72
  44. Connections for a Bi-unial Lantern                     73
  45. Beard's Circulating Water Tank                         75
  46. Quadruple Lantern Stand                                78
  47. Reading Lamp                                           79
  48. Roller Screen                                          80
  49. Portable Screen Stand                                  81
  50. Adjustment of the Light                                84

       *       *       *       *       *


{1}

OPTICAL PROJECTION

A TREATISE ON THE USE OF THE OPTICAL LANTERN

CHAPTER I

INTRODUCTORY

Lantern Projection, as commonly understood, may be broadly subdivided into
two branches: (A) The Projection of Lantern Slides, and (B) The Projection
of Scientific Phenomena, Opaque Objects, Microscopic Specimens, &c.,
usually referred to broadly under the heading of 'Scientific
Demonstration.'

To these two classes may perhaps now be added a third, viz. The Projection
of So-called Living Pictures, or, in other words, the Cinematograph. In the
earlier editions of this work both A and B were dealt with in the same
volume, but, as there are thousands who require to use a lantern for the
demonstration of lantern slides only, and who have no interest or concern
with Science Projection, it has seemed to the writer that the work might,
with advantage, be divided into two portions, Vol. I. dealing with slides
only, and Vol. II. with the various adaptations of the science lantern.
This present book therefore only deals with the exhibition of lantern
slides, and as such it will, I trust, be found to be of real assistance to
the ordinary user of the optical lantern, including clergymen,
schoolmasters, army and cadet officers, and others {2} who require advice
and instruction in the purchase or use of a lantern.

The essential parts of a lantern are: (_a_) A _slide-holder_ or _carrier_
to hold the slide; (_b_) a _lens_ to 'focus' it on the screen; (_c_) a
_condenser_ to converge the light upon slide and lens; (_d_) a source of
light or _radiant_ to provide the necessary illumination; and (_e_) a
_body_ or framework to hold the whole together. All possible variations in
choice of a suitable lantern relate to one or another of the above parts,
and will be treated of in turn; but, fortunately, we have this
all-important simplification that every ordinary English lantern slide is
the same _standard size_, viz. 3¼ inches square. Some Continental and
American slides differ in one dimension from the above, but not enough to
cause any serious difficulty, and the convenient English standard is being
gradually adopted throughout the world.

The varieties of slide-holders or carriers are therefore comparatively few
and are chiefly concerned with the question of rapidly and easily changing
the slides. The choice of a focussing lens or objective is mainly a matter
of the size of picture required, and the most convenient distance from the
screen for the lantern to be placed. Variations in condensers, which are
comparatively small, are usually only a matter of conforming these with the
size or type of objective to be used, and should be left to the
manufacturer's judgment. The question of a suitable radiant is partly a
matter of the amount of illumination required, and partly that of the
practical possibilities; for example, if electric current is available some
form of electric light is usually the most convenient, as well as the least
expensive, but where this is not the case, paraffin-oil, methylated spirit,
incandescent gas, acetylene, limelight, &c., are alternatives which all
have their uses and must be considered on their own merits.

Sometimes, as for example in the case of a travelling lecturer, a lantern
is required fitted with a range of lenses for {3} halls of different size,
and also with a variety of illuminants, and this in most lanterns can be
easily provided for.

The body is usually a matter of taste and price only, and may range from a
simple but efficient shell of Russian iron to an elaborate mahogany
instrument with a brass front, screw tilting arrangements and other
adornments; but of late years there has been a wholesome reaction against
unnecessary finish, and a simple metal body of some description is now
chiefly the order of the day. In the foregoing remarks the various parts of
a lantern have been mentioned in what I should consider the correct order,
starting from the slide and slide-holder, and so to speak building up the
rest of the instrument round these items; but I now propose somewhat to
vary the procedure and for convenience deal in detail first with the
Radiant, or _Illuminant_.

       *       *       *       *       *


CHAPTER II

THE ILLUMINANT

The first necessity for lantern projection is a strong light, and this can
be obtained from a variety of sources, the principal means in common use
being approximately in order of excellence as follows: paraffin-oil,
incandescent spirit, incandescent gas, acetylene, acetylene air blast,
oxyhydrogen (limelight), oxyether, and electric light in its various forms.
The ideal characteristics to be sought for are (1) great intrinsic
brilliancy; (2) minimum _size_ of luminous spot; (3) freedom from flicker;
(4) freedom from smell; (5) absence of any preponderating colour; (6)
cheapness; and (7) convenience. There is no question whatever as to which
of the available sources of light most perfectly combines all the above if
it is available, viz. the electric arc. If a current supply is in {4} the
building, this form of lighting easily excels all others, except possibly
in the matter of flicker, and even in this respect there is very little
fault to be found with it.

From all other points of view it is wellnigh perfect, inasmuch as it
provides an extremely concentrated and intensely luminous spot, of almost
perfect whiteness (if anything slightly bluish), no smell, comparatively
little heat, convenient and inexpensive. So great is the advantage of the
electric arc that attempts have been made to use it from accumulators in
places where a current supply is not available, but this cannot be
seriously recommended, except in special cases. Where an electric supply
is, however, available there can be no real choice, whether the lantern is
required for use in a large hall or a small class-room. The advantages of
using the arc are so great that no other method need be seriously
considered.

The one real objection that I have heard urged against it is due, curiously
enough, to its very perfection, and that is, that it lends itself to such
exceedingly sharp definition that any slight imperfection in the slide is
too faithfully reproduced on the screen, for which reason it is sometimes
recommended that the operator shall work with the objective the least
fraction out of focus; but this is a matter for individual taste and
judgment.

If, however, there is no possibility of using the electric current, one of
the other sources of illumination must perforce be adopted, and for a
_large_ hall this can only be limelight in one of its many forms, viz.
oxyhydrogen, oxyether, oxyacetylene, &c. As regards results on the screen,
this light compares well even against the electric arc, but it involves the
expense and trouble of compressed gas cylinders, or the infinitely worse
recourse to the now obsolete method of filling gas-bags.

Limelight is therefore now but little used in this country, as the majority
of large halls are equipped with the electric {5} current, and for smaller
buildings it is deemed unnecessary and too expensive.

ACETYLENE is undoubtedly the illuminant most in favour next to electric
light, as the light is brilliant enough to illuminate a picture 12 feet in
diameter at a distance up to, say, 30 feet from the screen, and this
suffices in a large majority of cases, and acetylene is comparatively
cheap, and reasonably simple to work.

INCANDESCENT-GAS is often employed for small class-rooms and is fairly
effective for a picture not exceeding 9 or 10 feet in diameter, and the
same can be said of the same type of burner heated by methylated spirit.

PARAFFIN-OIL is the poorest of all present-day forms of lantern
illuminants. The flame is large, impairing the definition, yellow in
colour, uneven in illumination, liable to smoke and smell, and barely equal
to incandescent gas in illuminating power.

It is therefore going gradually out of use in this country, but in
out-of-the-way places, especially abroad, it is sometimes the only
practicable light, and is therefore still employed from the best of all
reasons, necessity.

It is not the intention of the author to give precise working instruction
for all and every variety of the above illuminants as manufactured by
different firms. For such the reader must be referred to the directions
usually issued by the makers themselves, but a general description of the
various types offered for choice will not be out of place, and it will be
more convenient to begin with the poorest, viz. paraffin-oil, and finish
with the most perfect, the electric arc.

       *       *       *       *       *


{6}

CHAPTER III

PARAFFIN-OIL LAMPS, INCANDESCENT GAS AND SPIRIT BURNERS

[Illustration: FIG. 1.--Oil Lamp.]

There are several varieties of oil lamps on the market, but in practically
every case they take the same general form, a metal reservoir sliding in
grooves in the lantern body and holding approximately a pint of oil with
(usually) four wicks _nearly_ parallel, but slightly converging from rear
to front, these enclosed in a flame chamber of Russian iron, with _loose_
well-annealed ends of sheet glass and an adjustable reflector at the back,
or sometimes the reflector itself forms the rear end of the flame chamber.
The chimney must be tall and is now usually made adjustable, though I have
never been able to trace any real advantage from this complication {7}
(Fig. 1). The whole secret of obtaining the best results from these lamps
may be summed up--_good oil and perfect cleanliness_; and it is wonderful
what can be done when these points are properly attended to.

Care should be taken in trimming the wicks to see that no charred parts
fall down between the wick holders, but it makes little difference whether
the trimming is done with scissors or by rubbing with the finger. Special
lamp scissors are sold by all makers with a large flat on one side to catch
the portions cut off.

These lamps should be well rubbed over the last thing before use, as
paraffin-oil is apt to 'creep,' and the operator does not want to be told
that his apparatus is suggestive of a fried fish shop. In working with
these lamps it is difficult to avoid a dark streak down the centre of the
sheet, representing the space between the two centre wicks; to a certain
extent this can be obviated by adjusting the reflector, and in any case is
not very obvious when the slide is in place. Lamps constructed with either
three or five wicks are better in this respect, but the former are usually
considered to be too poor in illuminating power, and the latter are apt to
crack the sheet-glass ends by excessive heat.

INCANDESCENT GAS.--Incandescent gas burners do not need much description,
as they are practically similar to those in general use for house lighting.
They may be either of the erect or inverted forms, the latter being
preferable owing to the light being more concentrated, and a reflector is
provided to increase the illumination (Fig. 2).

These reflectors should be _spherical_ and so adjusted that the radiant is
in the centre of curvature, thus ensuring that the light from the reflector
passes again through the original source. If this point is not attended to,
we shall be dealing with essentially two sources of light instead of one,
to the detriment of the definition.

The same remark applies to every lantern illuminant {8} which is
supplemented by a reflector, and it is extraordinary how often it is
neglected by the manufacturer. Of course the opacity of the illuminant
destroys much of the efficiency of the reflector, and hence in the case of
incandescent gas mantles there is not much real gain in making use of them,
but with these comparatively weak illuminants every fraction tells, and the
reflector does not add much to the cost.

[Illustration: FIG. 2.--Inverted Incandescent Lamp.]

In light the inverted gas burner is very little superior to oil, but it is
whiter, slightly more concentrated, and freer from smell, and therefore to
be regarded as preferable if a supply of gas is available.

METHYLATED SPIRIT BURNERS.--Incandescent mantles heated by methylated
spirit are also largely used, and provide a light decidedly superior to gas
and nearly equal to acetylene. Some arrangement must be made for
volatilising the spirit and driving the vapour out under pressure, and the
most usual contrivance is somewhat as illustrated in Fig. 3.

In this apparatus the spirit is contained in a metal reservoir at the rear
and air pressure is provided by a pair of rubber balls and valves after the
manner of a medical spray. Sufficient {9} pressure having been obtained,
the liquid spirit is forced into a vaporising chamber immediately behind
the mantle, and a kind of miniature pitchfork, with its prongs wrapped in
asbestos wool, is soaked in spirit, and pushed over the brass fitting of
the burner in such a way that when lighted the flame heats the chamber and
volatilises the spirit. The burner can now be lit, and although the fork
burns out in the course of a minute or so, the heat from the mantle itself
is thereafter sufficient to vaporise the spirit as rapidly as required.
This lamp works exceedingly well in practice, but has one drawback, viz.
that it is possible to obtain too much pressure and squirt _liquid_ spirit
through the burner, when it naturally catches fire and may even run on to
the floor.

[Illustration: FIG. 3.--Methylated Spirit Burner.]

An accident of this sort is rare and usually harmless even if it does
occur, but an audience is easily frightened, and hence this burner should
only be used by _an operator with experience_. An altogether better
arrangement is that made by Messrs. Hughes of Kingsland and known as the
'Luna' Lamp (Fig. 4).

In this burner there is no pump and no volatilising chamber; {10} the
spirit is contained as before in a metal reservoir and a separate burner
underneath is used to keep this sufficiently hot to both vaporise the
spirit and provide the necessary pressure. The heat can be regulated by
means of an adjustable sheath to the burner, and a simple safety valve
provides against an excess of vapour.

I do not say that an accident of the sort previously referred to is
impossible even with this burner, but I have never heard of it happening,
and the lamp is certainly the best apparatus of its kind that I am
acquainted with.

[Illustration: FIG. 4.--Luna Lamp.]

INCANDESCENT ELECTRIC LAMPS.--Incandescent electric lamps of the ordinary
metal or carbon filament type are also frequently used in small
class-rooms, and should be mentioned here, as they provide approximately
the same illumination as a gas mantle, or in some cases rather better. It
will, however, be more convenient to deal with the question of electric
lighting as a whole in the chapter devoted to it.

It will suffice here to say that lamps are made for the purpose with a
special filament arranged to provide a concentrated light, the ordinary
type being almost useless in this respect, and that small battery lamps,
worked by a suitable accumulator, can also be used, but except under very
special circumstances are hardly worth the trouble of keeping the batteries
charged.

       *       *       *       *       *


{11}

CHAPTER IV

ACETYLENE

There is no doubt that at present acetylene holds second place to electric
light in popularity for optical lantern work. The light is good; not, it is
true, _so_ good as limelight or the electric arc, but still sufficient for
a picture up to 12 feet in diameter at a working distance from the screen
of not more than 30 feet, and this suffices for the large majority of
halls.

It has great advantages over limelight in convenience and cheapness,
although on both these points it must yield place to the electric arc,
always providing that current is available, and therefore it is chiefly
used in country districts and in gas-lit halls in large towns.

Acetylene gas is formed, as is well known, by the action of water upon
carbide of calcium, and the generators constructed for lantern work are
essentially the same in construction as for other purposes.

The alterations introduced are chiefly directed towards obtaining a light
as _steady_ as possible from a comparatively small generator, and,
secondly, towards the entire elimination of smell, which obviously is far
more serious in a lecture hall than, for instance, on a motor car. The
generators in most common use may be divided into two classes, i.e. those
on the gasometer principle in which the carbide is gradually lowered into
the water, and those in which the water is allowed slowly to gain access to
the carbide. A good example of the former is perhaps that made by Messrs.
Moss of Birmingham, though there are several others equally good, and clear
and explicit directions for working should be supplied by the makers. The
Moss Generator (Fig. 5) consists of a tall iron vessel A fitted with a gas
tap at bottom, this communicating {12} with a vertical iron tube within the
vessel. Into this container fits the inner bell or container B, divided
internally into two concentric portions entirely separated from each other,
but connected by the pipe P P and the tap T.

A guide inside the bell encircles the iron tube in the outer tank and
prevents rotation. Into the inner portion fits again the carbide-container
(shown separately on the left), which is locked when in place by giving it
a half turn, when a hook inside the bell engages with the lower edge of the
carbide container and prevents it from falling.

The carbide container is fitted with a series of shelves, and the contents
of a 2 lb. tin of carbide should be roughly divided among them; there is no
need to make any accurate division. The carbide used should be that known
as ½ inch mesh, and should be _pure_. That described as 'chemically'
treated is apt to give trouble by over-generation in these gasometers and
should be scrupulously avoided.

[Illustration: FIG. 5.--The Moss Generator.]

The carbide having been placed in the receptacles, these should be closed
by means of the loose flap and the whole pushed into the bell and secured.

Water should be poured into the outer vessel up to a mark on the iron tube,
and the bell placed in position. The lower tap being then turned on and the
upper one closed, air from the outer portion of the bell can gradually
escape by means of the iron tube and lower tap, and the bell gradually
sinks by its own weight until it is on the bottom, but still {13} no water
can reach the carbide, the air imprisoned in the inner portion of the bell
effectually excluding it.

The lower tap should now be connected by means of india-rubber or flexible
metallic tubing to the burner in the lantern (of which more anon), and the
upper tap on the generator turned on, the tap or taps on the burner being
likewise opened. The air from the inner portion of the bell can now escape
by the pipe P P into the outer part, and thence through the iron tube, and
out through tubing and jet, and as it does so water will rise in the
interior and attack the carbide.

In a few moments the burner can be lit; but the gas, being generated far in
excess of requirements, and filling both the inner and outer portions of
the bell faster than it can escape, lifts the latter until the carbide is
entirely out of the water, when in a few minutes generation ceases.

If the jet is left burning the bell will gradually sink again as the gas is
used up, and should thereafter maintain an automatic balance without
attention.

It can be turned off at any moment by simply closing the taps at the jet
or, better, the lower tap at the generator, when the bell rises
sufficiently to take the carbide out of the water; but if it is required to
leave the generator unlit for a considerable time, it is better to turn off
the tap on the top first. This causes the inner portion of the bell to fill
with gas which cannot escape, and as that in the outer part burns out, the
bell sinks to the bottom and remains there, the gas itself imprisoned in
the inner chamber excluding the water from the carbide. The exact
arrangement varies in different patterns of generator, but the above may be
taken as roughly indicating the action, and further information may always
be obtained from the maker or dealer.

_Emptying_ should always be done out of doors, as the odour of acetylene
gas is most objectionable, and for the same reason rubber tubes, &c.,
should be securely tied on, so that the slightest escape may be avoided.
{14}

If the exhibition has been a short one it will often be found that the
upper cells have not been affected by the water, in which case they may be
put back in the tin and used again, but it is not generally advisable to
put in less than the full charge to begin with as the weight of the carbide
plays a definite part in securing the smooth action of the apparatus. The
sludge should be thrown away (it forms a good manure for the garden) and
the entire generator thoroughly dried, otherwise rust will quickly appear.

[Illustration: FIG. 6.--The A.L. or 'Popular' Model.]

Theoretically one of these generators may be filled and left standing
indefinitely, but in practice it is not advisable, as the damp in the
atmosphere is apt to produce a very slow generation of gas, sufficient
often to cause a decided smell.

Of generators which act by admitting water to the carbide perhaps the best
known is the A.L. or 'Popular' Model (Fig. 6), this being, in fact, a
pattern designed for motor-car head-lights, but which answers well for
lantern work.

Its exact operation need hardly be described here in full detail. It will
suffice to say that the water gains access to the carbide by 'creeping' up
between two concentric copper cones, and in the event of over-generation
the pressure of the gas automatically checks the flow.

This generator is smaller than the gasometer pattern, and hence can be
recommended for portability; but in my experience the light is not quite so
steady, and the control rather less delicate, thereby causing on occasions
a perceptible smell, especially if left standing for a considerable time.

There are other types of generators, such as the 'Water {15} dropping'
variety, in which the water drips on to the carbide, and the reverse, in
which fine granulated carbide drops a little at a time into water; but
these types are not very frequently met with and need hardly be described.

It should never be forgotten that acetylene is an explosive gas and should
be treated as such. Searching for a leak with a lighted match, though
perhaps permissible when the operator knows his business, may be a
dangerous proceeding when the contrary is the case.

[Illustration: FIG. 7.--Acetylene Jet.]

Acetylene burners are generally of the 'Batswing' type, and are as a rule
four in number, mounted in a row with a reflector behind, each burner being
separately controlled by its own tap (Fig. 7). An acetylene flame is very
smoky, and care must be taken that the burners are not turned too high. A
nipple cleaner, consisting of a fine wire in a short handle, can usually be
obtained from any dealer, and is very handy.

Acetylene gas can also be used for lantern illumination in quite another
way, viz. by a blast from a blowpipe, in combination with either air or
oxygen, on to a special 'Pastille' provided for the purpose, or an ordinary
limelight jet can be used. These methods entail the use of acetylene _under
pressure_, and are so analogous to limelight that I shall for convenience
deal with them in the chapter devoted to that illuminant.

       *       *       *       *       *


{16}

CHAPTER V

LIMELIGHT AND THE ACETYLENE BLAST

The illumination possible with this light is almost unlimited, and for
really large halls it is, as remarked before, the _only_ substitute for the
electric arc. It consists essentially of a blowpipe flame, composed of
oxyhydrogen, oxyether, oxyspirit, oxy-acetylene, &c., or acetylene air
blast, heating to incandescence a block of lime, or other refractory
material, and the essential feature is that one at least of these gases
must be under _pressure_. Thirty years ago this was usually achieved by
storing the gas in rubber bags, and obtaining the requisite pressure by
means of heavy weights; but except in a very few outlying districts this
method has now been completely superseded by the use of compressed gas
cylinders. The earlier editions of this work contained very full directions
for manufacturing gas for storage in bags, but it is so exceptional now to
find an operator who uses this method that it seems hardly necessary to
devote much space to it, and the same may be said of automatic oxygen
'generators.' The present work will therefore deal chiefly with compressed
gas cylinders.

Most elaborate precautions are now enforced by the Board of Trade to ensure
the absolute safety of these, and any doubt existing from occasional
accidents of years ago may be promptly dismissed. Humanly speaking, an
accident nowadays _cannot_ happen, except by such wilful negligence on the
part of the maker or filler as would almost render the culprit subject to
criminal proceedings.

Compressed gas cylinders are painted a distinctive colour, oxygen for
example being black and coal gas or hydrogen red; the screw connections to
the pumps, and all nozzle {17} and regulator fittings, are made with a
totally different screw and therefore cannot be interchanged; the cylinders
themselves are bound by law to be reannealed and retested under hydraulic
pressure at regular intervals; the steel itself has to be of a guaranteed
quality; and, in fact, every possible risk is guarded against.

The most usual sizes of cylinders supplied for lantern exhibitions are
those containing 6, 12, 20, or 40 cubic feet, and are usually sent out in
wooden or hemp cases.

[Illustration: FIG. 8.--Oxygen Cylinder in hemp cover.]

Fig. 8 shows a 12-foot cylinder in its hemp case, the approximate size
without case being 22 in. by 4 in. This size cylinder will supply an
average limelight jet for just over two hours. The extra powerful jets as
used for cinematograph work or for illuminating a very large screen take a
good deal more, but for the usual apparatus as supplied for ordinary
lantern purposes this is a pretty safe figure.

A 12-foot cylinder is therefore the favourite size for a lantern exhibition
lasting from an hour to one and a half hours, as it leaves a fair margin
for gas used in adjusting the instrument, &c., and a 20-foot cylinder will
usually suffice for _two_ such exhibitions.

The price of gas per cubic foot varies with the size of the cylinder, being
less for large cylinders than for small ones, and the cost of transit is
also less in proportion--hence it is frequently an economy to hire a large
cylinder and retain it for several exhibitions. On the other hand most
suppliers charge a small rent if a cylinder is retained beyond a definite
{18} time, so this is a question to be decided by each user on its own
merits.

Alternatively, of course, cylinders can be _purchased_, and the question of
rent does not then come in; also gas is supplied a little cheaper in a
customer's own cylinder than if sent on hire. If purchase is decided on it
is frequently an economy to buy _two_, or two of each gas, if coal gas
cylinders are required as well.

[Illustration: FIG. 9.--Double Lever Key.]

The whole contents of the cylinders can then be used up without waste, as
if a cylinder should become exhausted during the course of a lecture, it is
only a matter of a minute or two to change over to the spare one, whereas
the compressors are required by law to empty out every cylinder returned to
them for refilling, and any remaining gas is thereby wasted.

It is extremely tantalising, to say the least of it, to find the pressure
gauge indicating that there is, say, 8 feet of gas remaining in a cylinder,
and to be compelled to waste this or else risk running short for the next
exhibition, and duplicate cylinders are the only way of avoiding the loss.

The cylinders are filled to a pressure of 120 atmospheres, or 1800 lb. per
square inch, and are closed by strong screw nozzles. The keys for opening
or closing these are of three types, viz. the 'T' pattern, 'Spanner'
pattern, and that known as the 'Double Lever' type. This latter is so made
that in closing the valve it shuts up to half its length and {19} opens out
to double the leverage when being used to _open_ the cylinder (Fig. 9). The
idea is to avoid the possibility, which has been known to occur, of the
cylinder valve being screwed down by a powerful wrist and defying the
efforts of the despairing lanternist to open it.

[Illustration: FIG. 10.--Fine Adjustment Valve.]

Cylinder nozzles are unfortunately not yet standardised, but those most
frequently met with in this country are those adopted by the British Oxygen
Company, both oxygen and coal gas cylinders being fitted with corresponding
_internal_ screws 7/8 inch diameter, those for oxygen being _right-handed_,
and those for coal gas _left-handed_, and in each case terminated at the
bottom by a hollow metal cone.

As such an internal screw cannot obviously be connected to a piece of
rubber tubing, some type of screw connector must be employed, and this may
take one of three forms: (1) A simple connecting nozzle, (2) a fine
adjustment valve, or (3) a regulator. The first is seldom used in practice
for lantern work, for the reason that the direct pressure of a full
cylinder (120 atmospheres) cannot be checked or controlled by a tap on the
jet, as the intervening rubber tubing would either burst or blow off, and
must therefore be regulated at the cylinder nozzle itself, and gradually
readjusted as the pressure diminishes.

To achieve this regulation with the ordinary cylinder key is difficult,
though possible to a careful operator, but for a slight extra expense a
combined nozzle and _fine adjustment valve_ (Fig. 10) can be obtained, and
regulation with this is {20} infinitely easier. The best plan of all,
however, is to use an automatic regulator, which not only reduces the
pressure so as to permit of the required adjustments being made at the
jet-taps, but also maintains a practically steady supply as the cylinder
empties, thereby obviating continual readjustments. Regulators are now so
inexpensive that they have come into almost universal use, and are
generally reckoned an indispensable part of a limelight lantern equipment.
The form of regulator in most common use is that usually known as
'Beard's,' having been originally designed and patented by Messrs. R. Beard
& Sons, though as the patent has now expired it is open to any firm to make
the same article if they desire.

[Illustration: FIG. 11.--Construction of Beard's Regulator.]

The construction of Beard's Regulator is shown in Fig. 11. The gas enters
from below into a rubber bag, C, from which it can emerge through the
nozzle.

Any accumulation of gas raises the bellows against the pressure of a spiral
spring pressing it down, and this brings into action an arrangement of
so-called 'Lazy Levers,' which in turn presses down a small conical valve
and closes the supply from the cylinder, this valve re-opening immediately
the pressure diminishes.

The outward form of this regulator is shown in Fig. 12, {21} which
incidentally also illustrates the usual form of connection to the cylinder,
referred to later on.

In Beard's Regulator the pressure at which the gas can be delivered is
determined by the strength of the spiral spring, and can only be altered by
changing this spring.

[Illustration: FIG. 12.--Beard's Regulator.]

In practice Beard's Regulators are supplied set to a low pressure for
ordinary mixed or 'blow-through' jets and for a higher pressure (14-16
inches) for 'injector' jets. At this latter pressure the rubber tubing used
must be fairly thick and strong and well tied on, and even so the taps of
the jet should not be turned entirely off unless the gas at the cylinder is
likewise turned off immediately afterwards. The British Oxygen Company make
a regulator which can be set to any desired pressure, but it is not quite
so delicate in its action as Beard's, and Messrs. Clarkson also make a
pattern regulator which is widely used and well spoken of. The attachment
of any of these fittings to the cylinder is a somewhat peculiar one, as
will be seen on reference to Fig. 10 or Fig. 12. The regulator or nozzle
ends at its lower extremity in a screw and cone, the latter being intended
to make a gas-tight connection with the internal cone on the cylinder, and
over this screws a loose wing piece with another outer screw, this latter
fitting the thread in the cylinder.

In making the connection care must be taken that the wing piece is not
screwed too far down the inner screw, or the cone will not reach down and
make a tight fit on its {22} seating; in its correct position the wing
piece when clamped down should leave a turn or two of its thread exposed,
in order to ensure that the cone does bed properly.

[Illustration: FIG. 13.--Regulator and Gauge.]

Care should be taken that the nozzle of the cylinder is free from dust
before attaching any of these fittings: the best plan is first to blow into
it, and finally wipe it round with the finger. Most professional operators
_hammer_ the wing piece home with a spanner or other convenient implement a
barbarous method and really unnecessary if the cones are in good condition,
but, nevertheless, almost always adopted in practice.

PRESSURE GAUGES.--These are useful in determining the amount of gas
remaining in a cylinder and are of a very usual type; they may either be
screwed on to the cylinder before commencing to work and taken off again to
screw on the regulator, or they can be supplied fitted to the regulator
itself, in which case they can be observed during the course of the
exhibition (Fig. 13). As the same gauge may be used for cylinders of
different sizes (though _never_ for those containing {23} different gases),
they simply register in atmospheres, and knowing that a full cylinder shows
a pressure of 120 atmospheres, the requisite calculation must be made to
determine how many cubic feet are unused.

In the case of oxygen cylinders an approximate idea of the amount of gas
remaining can be got by _weighing_ it carefully when known to be either
absolutely full or absolutely empty, and re-weighing it when information is
required. Oxygen weighs approximately 1.4 oz. per cubic foot, and this is
easily detected by an average scale. Coal gas is too light to be gauged in
this way.

GAS-BAGS AND GENERATORS.--It has already been remarked that there are two
alternative methods of obtaining gas under pressure for limelight purposes,
viz. gas-bags and generators (the latter for oxygen alone: there is no good
hydrogen generator that I know of). In both these cases the oxygen is
generated by heating a mixture of chlorate of potash and manganese black
oxide. In the case of gas-bags the gas is prepared beforehand and passed
through suitable purifiers into a rubber gas-bag. With a generator the
oxygen is evolved during the exhibition itself; but this method has never
come into very general use.

Coal gas or hydrogen is very seldom home generated; a gas-bag can, if
necessary, be filled a few miles away and brought full to the place of
exhibition, or filled on the spot if gas is laid on; or, failing this,
acetylene or ether, or even methylated spirit may be utilised instead.

The bags in use are placed between double pressure boards (if _both_ gases
are required under pressure) and weights sufficiently heavy placed on the
top (Fig. 14), or with a 'blow-through' jet only the oxygen need be stored
in a bag and the coal gas used from the supply main.

Cylinders have, however, so universally superseded these appliances, that
space is hardly warranted in fully describing them, especially as any
operator wishing to adopt {24} the process can obtain full directions from
any responsible dealer.

LIMELIGHT JETS.--These are of three general types, viz. the 'Blow-through,'
the 'Mixed,' and the 'Injector.'

Of these the 'Blow-through' is now very little made, having been largely
superseded by the 'Injector' pattern, but, as there are hundreds in common
use in this country, they cannot yet be regarded as a thing of the past.

[Illustration: FIG. 14.--Gas-bags.]

The exact design of this jet varies considerably, but all are alike in
this, that a jet of coal gas is burned at the orifice of a more or less
open nozzle, and a stream of oxygen _blown_ _through_ it on to a cylinder
of lime which it thereby renders incandescent. Fig. 15 represents the
various designs chiefly adopted for this jet, that marked A being perhaps
the most usual, though C is also frequently met with.

In light-giving power there is not much to choose between the various
types; probably D on the whole is the best in this respect, but so much
depends upon the exact position of the two nozzles, and the _smoothness_ or
otherwise of that {25} provided for the oxygen blast, that exact
comparisons are difficult.

[Illustration: FIG. 15.--'Blow-through' Nozzles.]

'Blow-through' jets are the weakest form of limelight as used at the
present day, and may be taken roughly as some 50 per cent. better than
acetylene, or in other words, sufficient to illuminate a 12-foot picture at
a distance of some 40 to 50 feet; but their advantage is, or was, that they
only required one gas (oxygen) under pressure, the coal gas supply being
obtained from the ordinary house main.

[Illustration: FIG. 16.--'Blow-through' Jet.]

This advantage is now shared by the more recently introduced 'Injector'
jets, which give a far better light, and have therefore rendered the
'Blow-through' type nearly extinct.

The general construction of a 'Blow-through' jet is shown in Fig. 16, and
it will be seen that a short vertical spindle is {26} provided to carry the
lime cylinder, and that this can be rotated from the back by means of
bevelled gear wheels, which at the same time screw the spindle up and down.
A lime cylinder of the usual pattern being placed on this spindle can be
rotated from time to time to expose a fresh surface, as that in use
gradually becomes 'pitted' by the blast, while the screw provides
sufficient vertical movement to ensure that a complete rotation does not
bring round the same position again.

Some arrangement is also generally provided by which the distance between
the lime spindle and the jet can be adjusted. The exact position of this
does not matter within a reasonable margin, but limes vary in size, and
'Pastilles,' and other substitutes for limes, which will be referred to
later, vary still more, at any rate as regards this adjustment. The average
distance which gives the best result is usually about half an inch, and
once set need not be altered with that particular jet unless a lime of
different size is employed; minor variations due to limes being drilled
slightly out of centre, &c., do not seriously matter.

There is no accepted rule for colouring jet-taps in accordance with the
cylinders, and although jets are sometimes met with painted in this way,
_i.e._ red for coal gas and black for oxygen, it is more usual to find coal
gas taps _black_ and oxygen _bright_, or sometimes both black or both
bright. Care must therefore be taken that the right cylinder is connected
to the right tap on the jet, but there should be no difficulty in telling
which is which, and fortunately any mistake, even if it be made, is quite
harmless.

THE MIXED-GAS OR DOUBLE-PRESSURE JET.--This jet is fundamentally different
from the 'blow-through' form, inasmuch as the two gases are combined in one
mixing chamber before combustion, and burn in their correct proportions at
one nipple.

It is usually stated that this jet necessitates both gases being under
equal or approximately equal pressure, but this {27} is not literally
accurate, and I have given many a lantern exhibition with one of these
jets, using coal gas from the ordinary supply, and oxygen from a cylinder.
To use a mixed jet in this way needs care, as a very slight excess of
oxygen puts the light out with a 'pop' which, although not dangerous, is
disconcerting, while the light obtained under these conditions is very
little better than with a 'blow-through' jet, and far inferior to the
'Injector' jets to be described next.

[Illustration: FIG. 17.--Mixed Jet.]

[Illustration: FIG. 18.--Mixed Jet, Gwyer pattern.]

The mixed-gas jet is intended then to be used with both gases under
pressure, and is the _only_ jet to be seriously {28} considered in cases
where a really powerful light is required. The power of this jet is indeed
almost unlimited, and those made with large bores, such for example as used
for cinematograph work, provide a light amounting often to some two or
three thousand candles, and consume an enormous amount of gas; but the
ordinary pattern, with a nipple of one-twentieth to one-sixteenth of an
inch bore, and using some 5 feet of each gas per hour, or perhaps slightly
more for the coal gas, will suffice for all ordinary work.

[Illustration: FIG. 19.--Mixing Chamber of Jet.]

The mixed-gas jet, like the 'blow-through,' is made in many forms, but
these may be roughly divided into two main types, viz. those with small
mixing chambers immediately below the nipple (Fig. 17), and those with
larger chambers in the horizontal part of the jet as in the 'Gwyer' pattern
(Fig. 18).

The construction of the mixing chamber itself varies also, but that
advocated by my father, the original author of this work, is generally
followed, the chamber being packed with alternate discs pierced as in Fig.
19, which ensures a thorough mixture of the gases. A layer or two of gauze
is often introduced as well by way of further improvement. The distance
between the lime and nipple is much less than with the 'blow-through' jet,
and the adjustment has to be more exactly made. About 1/8 inch is
approximately correct for a jet of moderate power, and rather more for a
bigger bore; also care must be taken to turn the lime frequently, as the
latter 'pits' pretty quickly with these jets, and if it is neglected the
jet may spurt back out of the hole, which is gradually formed, and crack
the condenser. {29}

There is still an erroneous opinion extant that these jets are dangerous,
and if the operator is working with the now obsolete gas-bags it is
certainly a fact that an accident in careless hands is _possible_; but with
cylinders there is, so far as I know, no possibility even of an accident
under ordinary conditions.

It is true that if too much oxygen is turned on the jet may suddenly go out
with a loud snap or pop, and this is in reality a miniature explosion in
the mixing chamber; but it can in any case hardly be serious enough to
matter, though I have found after such a snap that the gauze packing,
inside the chamber above referred to, has been pierced right through, and,
when first lit afterwards, the jet has for a few minutes burnt with a
characteristic green flame, denoting the presence in the gas of fine copper
or brass particles.

To obtain a good light with these jets, and in fact with _all_ jets, great
care must be taken that the nipple is absolutely smooth, otherwise the
flame is bound to hiss. The simplest plan is to slightly roughen a suitable
sized needle with emery paper and to burnish the inside of the nipple from
time to time with this. Especially if there has been one of the 'snaps'
referred to is it desirable to see that the inside of the nipple is
thoroughly smooth and polished.

MANIPULATION OF THE MIXED-GAS JET.--On this point there is not much to be
said. A good hard stone lime must be used--'soft' limes are useless for
this jet--and the coal gas flame should be lit first, and the lime
thoroughly heated with this before the oxygen is slowly turned on. As the
oxygen increases the flame will gradually disappear and the light increase,
until it is at a maximum for that particular amount of coal gas. This
latter can then be turned on a little more, and more oxygen passed to
balance it until the jet begins to 'roar,' when we are getting the maximum
light for that particular sized nipple. When the two gases are, however, in
the proper proportion to give the best light, there will always be a slight
excess of coal gas flame visible playing about the lime. {30}

THE INJECTOR JET.--This is essentially a mixed jet, and in outward
appearance differs but little from one of the ordinary type (Fig. 20), but
is so constructed that the pressure of oxygen 'sucks' coal gas into the
mixing chamber, and so obviates all necessity for the latter being under
pressure.

With this jet there is little or no danger of the jet 'snapping' out
through a surplus of oxygen, as the greater the flow of this gas, the
greater the suction on the coal gas side.

The light is not quite equal to a good mixed jet, but very nearly so, and
therefore this jet is deservedly gaining in favour every day.

[Illustration: FIG. 20.--'Injector' Jet.]

One point must be noted: the oxygen itself must be under greater pressure
than with the ordinary mixed jet if the best light is to be obtained, and
therefore a special regulator must be used, or one of ordinary type
modified (which can easily be done by the maker), and rubber connections
must be securely tied both on to jet and regulator, as the pressure
required to work this jet to advantage, while not enough to burst a rubber
tube, is enough to blow it off an easy fitting connection.

THE OXYETHER LIGHT.--This is practically similar to the oxyhydrogen, except
that ether vapour is used in place of the hydrogen or coal gas. The method
adopted consists essentially of passing a stream of oxygen through a vessel
packed with some porous material (such as cotton wool or cotton gauze)
which is saturated with ether. The oxygen {31} becomes saturated with ether
vapour, and the mixture is then used in place of the coal gas supply in a
double-pressure jet, an additional supply of free oxygen being still
required through the ordinary oxygen tap.

The arrangement is cheap, as it dispenses with the necessity for a coal gas
cylinder, and effective, as the light is little, if at all inferior to the
oxyhydrogen, but differs from the latter in this, that with careless
handling an accident is _possible_.

In competent hands there is no danger, and I have used ether saturators
myself scores of times without one single contretemps; but it should _not_
be entrusted to any chance amateur.

The use of the ether light has a curious history. In the earlier days
before the proper construction of ether saturators was understood, and
gas-bags were still in vogue, it was largely condemned on the score of
danger. Modern improvements in apparatus rendered it perfectly safe against
anything but gross carelessness or bungling, and the London County Council
and other similar bodies immediately supplied it broadcast to elementary
schools (in disregard of warnings offered by myself and others), where it
was often entrusted to incompetent operators or even senior boys. So far as
I know no serious accident ever resulted, a pretty conclusive proof that
the light is really safe, but in time the London County Council realised
that the universal adoption of this illuminant was not advisable, and I
believe _now_ prohibit it altogether in halls licensed by them for
entertainments.

In time, no doubt, they will learn to adopt a sane policy between the two
extremes, but at present the official attitude in many localities has
placed ether saturators out of the running, and before purchasing one the
would-be operator should ascertain that he will be allowed to use it.

Ether saturators as made at the present day may be divided into two
principal patterns, viz. those in which saturator and jet are combined in
one piece of apparatus {32} which fits bodily into the lantern, and
saturators which are used outside and connected by means of tubing to any
ordinary oxyhydrogen double-pressure jet.

Both forms have their advantages and disadvantages; the first pattern tends
to become too warm from its position in the lantern and generates ether
vapour too quickly, while the second has the fault of becoming too cold
(owing to evaporation of the ether) and therefore not vaporising quickly
_enough_.

[Illustration: FIG. 21.--'Gridiron' Saturator.]

Writing at the present date, when manufacturers are slowly beginning to
resume their normal occupations after the stress of war work, it is
impossible to say exactly what models will or will not be made, but I will
mention one typical example of each pattern as made in pre-war days.

The first of these is the 'Gridiron' (Fig. 21), adopted largely by the
London County Council in the days I have referred to, and certainly one of
the best designed saturators ever put on the market.

In the 'Gridiron' saturator there are three taps: two at the {33} rear and
one in front, between the saturator and the mixing chamber. Between the
rear taps is the inlet for the oxygen, which divides into two channels,
that on the left passing upwards through the U tube shown in the
illustration (the corresponding tube on the right is merely a dummy), and
thence through the saturator and out through the horizontal tube and tap
into the mixing chamber, whence the saturated stream of oxygen finally
passes to the nipple, and the combination burns with a whitish flame
closely resembling that produced by coal gas.

[Illustration: FIG. 22.--'Pendant' Saturator.]

The other channel for the oxygen is to the right, down the vertical tube
shown there (the lower vertical tube on the left is also a dummy),
underneath the saturator, and finally coming up into the mixing chamber
from below, transforming the white flame into an intensely hot blowpipe
exactly as it does with a coal gas jet. The front tap controls the supply
of saturated ether to the mixing chamber, and whereas at first a good
stream of oxygen is needed to pick up enough ether, by degrees as the
instrument warms in the lantern, the oxygen passing through the saturator
can be cut off entirely, and even then the front tap must be gradually
closed down to prevent the hot ether coming off too fast.

There is a disagreeable feeling of 'sitting on the safety-valve' in doing
this, but in reality the pressure is never likely to become great enough to
cause danger.

Of saturators for use outside the lantern the best-known is probably the
'Pendant' (Fig. 22). With this instrument the oxygen supply is connected to
the inlet marked A; B goes {34} direct to the oxygen tap of any ordinary
mixed-gas jet; while C, from whence issues the saturated stream, is
connected to the coal gas tap of the jet. Whichever pattern is used, the
essential thing is to keep a good supply of oxygen well saturated. If the
lime becomes incandescent without any free oxygen, or it is found that this
requires gradually turning off, it indicates that the saturation is
becoming defective, and to continue is to risk the jet snapping out. In the
case of an outside saturator such as the 'Pendant,' this may even blow off
the connecting tubes with a loud report, though no worse accident is likely
to happen, and for this reason an outside saturator should be placed _as
close_ to the jet as possible, so that the rubber tube may be kept short,
and incidentally this keeps the saturator warm and accelerates
vaporisation.

As ether vapour usually contains a certain amount of moisture which does
not vaporise to any great extent, this gradually accumulates and the
capacity of the instrument becomes reduced. It is therefore usually
necessary to return a saturator to the makers every now and again for
repacking.

The only real danger with a modern saturator is not in using but in
_filling_. This should be done if possible in the open air, and at any rate
never near a light. Ordinary sulphuric ether of specific gravity 720-730 is
usually considered the best, and a quarter of a pint will keep an ordinary
small-bore jet going for nearly two hours.

More precise directions are usually sent out by the makers, and as the
various patterns of saturator in use are pretty numerous, it would be
useless here to attempt more detailed instructions for working.

OXY-ACETYLENE JETS.--Any good mixed gas jet may be used with acetylene
instead of coal gas, provided that it is under pressure more or less
corresponding to that from an oxygen cylinder, and at the present day there
is no difficulty in obtaining this, in civilised countries at all events,
by {35} means of compressed or, to speak more correctly, 'dissolved'
acetylene cylinders, referred to later on.

With an 'Injector' jet there is no need for the acetylene gas to be under
pressure at all, and a simple generator such as described on page 12 will
answer perfectly, though in practice very seldom used. With such a
generator the pressure is so low that in many cases the jet will not even
burn until _some_ oxygen is turned on; but this introduces no real
difficulty, as with a good 'Injector' a snap is practically impossible,
provided the generator is large enough to evolve sufficient acetylene. It
is far better in every way, however, to use the acetylene from a cylinder,
just as with coal gas. Only in this case the cylinder is completely filled
with a porous material, and this again filled with liquid acetone or other
suitable fluid, in which the acetylene is dissolved as rapidly as it is
pumped into the cylinder.

To compress acetylene in the ordinary way is neither safe nor practicable;
but these 'dissolved' cylinders are now used extensively for both
oxy-acetylene welding and motor car lighting, and may be entirely relied
upon.

The D.A. (Dissolved Acetylene) Company were the pioneers in this country of
the industry, and their methods of business are peculiar and ingenious. The
user is requested in the first place to purchase a cylinder, and he then
becomes the owner of _a_ cylinder, but not of one _particular_ cylinder. A
list is supplied to him of various depots in the country where the
Company's cylinders are stored, and when empty he can, on payment of a
fixed sum, exchange his empty cylinder for a full one, which then becomes
_his_ cylinder _pro tem_.

This saves the delay and expense of returning a cylinder to London, and
incidentally clears the customer of any question of deterioration, this
being obviously covered by degrees with each individual exchange. The
system was first introduced in connection with the lighting of cars and
only applies to the standard size for this purpose, viz. 20 cubic feet
capacity, {36} but as this is, on the whole, the most convenient size for
lantern work also, the limitation is not a disadvantage. The arrangement is
also in vogue to a less extent with cylinders of 6 feet capacity (a size
sometimes used for motor _cycles_), but the depots of exchange are at
present far fewer for this size.

The oxy-acetylene blast is much _hotter_ than the ordinary oxyhydrogen, and
therefore produces a more intense light. I have therefore used it with
success on occasions when even the ordinary limelight would fail, and the
choice has lain between an oxyhydrogen jet of enormous bore (and, of
course, corresponding consumption of gas), and the oxy-acetylene.

For this very reason great care must be taken only to use the hardest
limes, and even then to use the lime-turning movement frequently, or the
lime will pit or crack and a broken condenser follow.

THE FALLOT ACETYLENE LIGHT.--This light consists of a jet of acetylene
under pressure, without oxygen, but producing its own _air blast_ from the
atmosphere by suction, much as the 'Injector' jet does, but the reverse way
round.

The light is better than with an ordinary acetylene jet, though not quite
so good as with a 'blow-through' jet; but as it only requires a cylinder of
dissolved acetylene, or even a 'Pressure' generator, it is fast coming into
favour.

The peculiarity of the Fallot apparatus is that, instead of providing a
direct beam of light in the direction of the screen, it projects the beam
_backwards_ on to a concave mirror, and it is the reflected light from this
that is used (Fig. 23).

Instead of a lime is used a spherical 'Pastille' of peculiar composition,
and before use each pastille must be burnt off exactly like an incandescent
gas mantle, after which it is extremely fragile and difficult to handle.

To use this illuminant one lens of the condenser must be removed, the
curvature of the mirror taking its place, and it will be seen at once that
the pastille itself will get in its own {37} light and throw a shadow,
which actually happens, but it is hardly perceptible unless specially
looked for.

[Illustration: FIG. 23.--Fallot Air Blast.]

A complete Fallot Air Blast Outfit, with cylinder, fine adjustment valve,
pressure gauge and burner, with two spare pastilles, is shown in Fig. 24,
but if preferred a regulator, such as previously described for oxygen, can
be used instead of the fine adjustment valve.

[Illustration: FIG. 24.--Fallot Air Blast, and Cylinder.]

{38}

FALLOT OXY-ACETYLENE BLAST.--This is similar to the foregoing, utilising
oxygen from a cylinder instead of air, and the light is equal to a powerful
limelight, and may be considered as an efficient substitute, though for
_long range_ work the shadow before alluded to becomes more noticeable (for
optical reasons which need not be here discussed). The Fallot Company also
make a special 'Pressure Generator' which can be used instead of a D.A.
Cylinder; but my experience of this so far is that, although perfectly
safe, the blast from it is a little unsteady as compared with a cylinder.

LIMES AND ACCESSORIES.--Limes for Optical Lantern work are usually supplied
in the form of cylinders, the 'ordinary' size being 7/8 inch in diameter
and about 1½ inches long, with a hole drilled longitudinally to take the
lime pin. Extra large limes up to 2 inches in diameter are supplied for
more powerful jets.

So-called 'soft' limes used to be recommended for 'blow-through' jets as
giving a better light than 'hard' limes, but the advantage, if any, is very
little, and these limes are now very seldom heard of, possibly because
'blow-through' jets themselves are becoming less and less used, and 'soft'
limes will not stand the heat of a mixed or 'Injector' jet for long.

'Hard' limes are turned out of the hardest stone lime, and must be kept in
sealed tins until used, as they rapidly disintegrate when exposed to the
air. There are one or two quarries known to provide the best lime for
lantern purposes, and the various good brands on the market practically
have the same origin as regards raw material, though called by different
trade names; and the 'Hardazion' (hard as iron) limes, placed on the market
some years ago by a well-known wholesale firm, to be countered shortly
after by the 'Hardastil' (harder still) brand, are, I take it, legitimate
though amusing instances of phonetic advertisement.

Even the best of limes is liable to crack under the heat {39} of a powerful
jet, and so a pair of lime-tongs should always be provided, and there is
nothing better than the simple form shown in Fig. 25, and which is, or
should be, sold by all dealers.

[Illustration: FIG. 25.--Lime-tongs.]

SUBSTITUTES FOR LIMES.--A good substitute for lime, that will give the same
light, stand heat equally as well, and _not_ deteriorate if exposed to the
atmosphere, has long been sought for, and some of the more recently
discovered refractory materials are more or less satisfactory. 'Mabor'
limes, for example, belong to this class, and so do some of the
'pastilles,' which before the war came chiefly from France and to a less
extent from Germany.

       *       *       *       *       *


CHAPTER VI

THE ELECTRIC LIGHT

The electric current provides _the_ light for an optical lantern, though it
may take various forms, such as the incandescent glow-lamp in some shape or
other, the comparatively new Ediswan 'Pointolite' lamp, the enclosed arc,
and the open arc. This little book is not a treatise on electricity, but a
few elementary notes may not be out of place, and may be of assistance to
the non-technical lanternist.

The first point then to be considered in adopting the electric light for
the purpose of lantern projection is the character of the supply, and the
information required may be summed up thus: (1) _E.M.F._, _voltage_, or
_tension_ (these three expressions having exactly the same meaning); (2)
_ampèrage_ or amount of current available; (3) whether current is (_a_)
_continuous_, _constant_, or _direct_ (again three words meaning {40} the
same thing), or (_b_) _alternating_. The E.M.F. or tension corresponds to
_pressure_, to use the mechanical analogy of a water pipe, and the
_ampèrage_ to volume, and the voltage of the supply currents in this
country are usually between 100 and 250 volts. Private lighting sets are
frequently as low as 50, and current derived from accumulators may be
anything from a few volts and upwards. _Power_ currents, such as commonly
employed for tramways, &c., are usually about 500 volts, but the use of
these currents for lighting purposes, though practicable, is not to be
advocated.

Ampères and volts are convertible terms in a sense; that is to say, a
current of 10 ampères at 100 volts requires the same horse-power to
generate it as one of 5 ampères at 200 volts, or 20 ampères at 50 volts,
but they are by no means convertible as regards their _efficient_ use for
our purpose. The ampères used multiplied by the number of volts give the
total power consumed in _watts_, and 1000 watts used for one hour represent
1 _unit_ as charged for on our dreaded lighting bills. The current
available from a public supply may be said to be unlimited so far as our
purpose is concerned, and the amount actually used depends only on the
total electrical resistance of our circuit, and this is measured in _ohms_,
the three factors, viz. volts, ampères, and resistance, being connected by
the well-known and simple equation C = E / R, C representing the current in
ampères, E the tension or E.M.F. (electro-motive force) in volts, and R the
resistance in ohms. The total current we _can_ use, however, is limited by
the size of the cable laid on in the building, and this is automatically
safeguarded (or should be) by the _fuses_, which consist, as is generally
known, of thin wires or strips of tin or lead fixed on a fuse board in an
easily accessible place, and which melt directly the current exceeds a safe
amount in ampères. Whatever method of lighting we use therefore, _enough_
resistance must always be kept in the circuit to ensure {41} that no more
current can pass than has been provided for, and in the case of an arc lamp
this usually means a _resistance_ or rheostat being retained in the circuit
in addition to the arc itself, through which the current is passed and
absolutely wasted, though fortunately the waste in money is negligible, and
for reasons to be discussed later such a resistance is necessary with an
optical lantern arc lamp in any case.

In the case of a glow-lamp, the entire resistance is provided by the
filament of the lamp itself, and that is why an ordinary metal or carbon
filament lamp, for say 200 volts, has to be manufactured with an extremely
long and slender, and therefore fragile, filament, while with an ordinary
pocket-torch, which is usually supplied with current from a dry battery of
some 3 or 4 volts only, the filament can be short and thick.

Speaking generally, glow-lamps on a low voltage current can be made more
efficient than on a high one, and are also longer lived for very obvious
reasons; but, on the other hand, the transmission of current over long
distances is cheaper the higher the tension, as for a given number of watts
the ampèrage is less, and therefore smaller cables can be employed. On the
whole, then, currents of 200 to 250 volts have during recent years become
more common than 100, in spite of greater difficulties in making the lamps;
but occasionally one finds a hall where two or more lamps are wired in
_series_, two 100-volt lamps for example being wired together in series on
a 200-volt circuit. If we are using current for our lantern from an
ordinary lamp socket, this is a possibility that must be borne in mind.

The same considerations, viz. the economy of transmitting power at high
tension and of _using_ it at a lower one, have been mainly responsible for
the rapidly increasing number of alternating current circuits now met with,
especially in sparsely populated districts. An alternating current main is
one in which the current reverses its direction, usually in this country
50, but sometimes 60, 80, 90, or even 100 times {42} per second (there
being unfortunately in Great Britain no standard 'Periodicity' or number of
cycles per second), and for technical reasons which need not be entered
upon here, with these alternating currents the tension and ampèrage can be
mutually converted by means of _transformers_, so that current can be
transmitted at so high a tension, for instance, as 10,000 volts, and used
at a voltage of 50 or 100 or whatever is required, the ampèrage available
being increased in inverse ratio as the tension is decreased. The same
ready power of transformation unfortunately does not apply to the
continuous current, or alternating currents would probably never have been
heard of, but as it is they are very common. For glow-lamps it is
immaterial which current is available, but for arc lamps the continuous is
much to be preferred, though both can be utilised.

With these initial remarks, I will now take in order of illumination the
various methods of utilising the electric current for optical lantern work.

THE ELECTRIC GLOW-LAMP.--The ordinary metal filament lamp is not very
suitable for lantern work, the light not being sufficiently concentrated,
but from what has already been said, it will be evident that this method of
lighting is more suitable where currents of low voltage are available.

An extremely good and intense light can be obtained from a comparatively
small battery of accumulators, which can easily be carried in the hand, and
a short and thick metal filament lamp, similar to those supplied with a
powerful electric torch; and this arrangement is actually used to some
extent by travelling lecturers, but the mess and trouble of keeping the
accumulators in order have prevented the method being generally adopted.

When _alternating_ current is available such a lamp will work well with a
transformer to step down the voltage to the required degree, and the
arrangement is simple, cheap, and efficient, and produces a light at least
equal to that from {43} acetylene. In comparatively small halls, where the
current is alternating, this is undoubtedly the best method of working, as
it is simpler than the arc and amply brilliant enough for all practical
purposes.

With the continuous current the problem is not so simple, as transformation
of voltage is not an easy matter, and a glow-lamp on; say, a 200-volt
circuit involves a long and fragile filament, which it is difficult to
arrange in a small space.

Many years ago the Ediswan Company produced a 'Focus' lamp for the purpose,
with the filament arranged in the form of a square grid, and this lamp gave
a light of about 100 candles, and was fairly successful for a small room.
More recently the Osram Company introduced a similar lamp with a metal
filament arranged somewhat in the form of a cone, and this lamp also
sufficed for a small class-room. It was, I believe, made in Germany and was
practically unobtainable during the war. I understand the Osram Company are
at present arranging to manufacture it in this country, but up to the time
of writing it has not made its appearance.

None of these lamps worked direct on a public lighting circuit can be
regarded as really satisfactory, as it has been found impossible so far to
get a _concentrated_ light; the 100-volt lamps have, of course, been
superior to those made for 200 or 250, but they are all for lantern
purposes far behind low voltage lamps, which are really good when a
suitable current can be obtained.

THE POINTOLITE LAMP.--This lamp produced by the Ediswan Company is in
reality a miniature arc with tungsten electrodes in a highly exhausted
vacuum bulb. To attempt a technical description would be beyond the scope
of this book; it will suffice to say that the action depends upon the same
principle as the various wireless vacuum valves or the Coolidge X-ray tube.

This lamp requires a peculiar starting device which is supplied with it,
and gives a good, intense, and concentrated {44} light, not equal to the
ordinary arc or to limelight, but comparing well with any other form of
illuminant. It can only be used with the continuous current.

THE NERNST LAMP.--This lamp at the present moment is practically
non-existent in this country, having been made exclusively in Germany. Also
as recent improvements in metal filament lamps have rendered it almost
obsolete for ordinary lighting purposes, it is, I think, very doubtful
whether it is still manufactured even in that country, and hence I do not
propose to waste space in an extensive description.

It will suffice to say that the lamp consists of one or more straight rods
or filaments of a refractory material, which are semi-conducting to the
electric current when hot, but non-conducting when cold. To commence with
the filament must be heated, and in the lamps as supplied for lantern work
this is usually done by means of a spirit lamp, which can be removed
immediately the current begins to pass, as the filament is thereafter
maintained at a white heat automatically.

A three-filament Nernst lamp gives as much as 1000 candles, but it is
extremely hot, and the light rather diffuse. The filaments are also very
fragile, so on the whole the lamp was never very much in favour; but on the
other hand it consumed very little current, and could be worked from any
ordinary house lighting main, points which led to its adoption in certain
cases.

THE ELECTRIC ARC.--We now come to _the_ light for optical lantern work, the
brightest, the most concentrated, the cheapest, the easiest to work, in
fact, the illuminant which combines all the virtues and but few drawbacks,
but of course requires one indispensable condition, viz. electric current
laid on. This current may be of any voltage from 70-250, or even higher; it
may be continuous or alternating, though the former is to be preferred; and
it requires a cable for _at least_ 5 ampères, and for a large hall 10 or 12
ampères.

The simplest form of arc lamp for lantern purposes is the {45} hand-fed
type as illustrated in Fig. 26. The essential feature is the pair of carbon
rods, the remainder of the apparatus consisting of mechanical adjustments
to 'feed' these as they burn away, and to accurately maintain them in their
proper positions and in the optical centre of the lantern. Just because the
electric arc provides so small and concentrated a light, it is of extreme
importance that the centring should be exact; and hence mechanical
movements are usually provided for this which are unnecessary with other
illuminants.

[Illustration: FIG. 26.--Hand-fed Arc Lamp.]

The whole question of optical adjustments has, however, been left over for
a future chapter, as it more or less applies to whatever illuminant is
used.

The illustration shows a lamp arranged for continuous current, the upper
carbon, which must be connected to the _positive_ wire, being larger than
the lower (the negative), and very slightly behind it. The light from a
continuous current arc lamp comes chiefly from this upper or positive
carbon, {46} which 'craters' as it is used, and this arrangement has the
effect of radiating the light in the direction required (Fig. 27).

The positive carbon is usually of the 'cored' type, that is provided with a
core of softer carbon, as this assists the 'cratering' action, while the
negative is generally used 'solid,' that is homogeneous right through.

The arc has to be 'struck' in the first place by touching the carbons
together for a moment by the mechanical means provided, and then separating
them to the working distance, which is approximately 1/8 inch. They must
then be maintained at that distance by 'feeding' as they slowly burn away,
and this 'feeding' in arc lamps for lantern work is usually done by hand,
as in the lamp illustrated in Fig. 26, but may be done by an automatic
arrangement, as will be described later.

[Illustration: FIG. 27.]

The current is really carried across the arc by _convection_, or in other
words conducted by a bridge of white hot carbon particles, which
continually stream across from the positive carbon to the negative, and
this bridge, while conducting the current, interposes a very considerable
_resistance_ (otherwise it would not of course become hot).

A certain potential or tension is therefore necessary if a given current is
to be maintained, and this potential has to be greater the longer the arc
and also (though not in direct proportion) the smaller the carbons.

When, however, everything is in the best proportion, _i.e._ length of arc,
size of carbons, and current passing, the potential at the arc lamp
terminals required is approximately 45 volts, and this may be taken as a
fixed figure for any current.

The length of arc to give the best results may also be taken {47} as
approximately fixed at 1/8 inch, and the _variable_ factor for different
currents as required is provided by altering the sizes of carbons employed.

The error must not be made, however, of assuming that an E.M.F. of 45 volts
is sufficient to work an arc lamp, as the minimum in practice is at least
65 volts, and 100 or even 200 volts are advantageous.

I have come across more than one private generating installation where the
innocent owner has put in a dynamo for 45 or 50 volts, depending upon some
carelessly written statement that this is sufficient.

_Why_ a higher E.M.F. is required can be simply explained.

Take for instance an average hand-fed arc lamp as used for lantern work and
consuming, say, 10 ampères.

Take also, as a fact, the statement given above that the necessary E.M.F.
at the actual terminals of the arc lamp may be accepted as a constant at 45
volts, and reverting to the equation given on page 40, C = E / R, and
substituting these figures we get--

  Current (10 ampères) = E (45 volts) / R (Resistance of Arc).

It is therefore obvious that under these exact conditions the resistance or
back E.M.F. of the arc, as it is termed, must equal 4.5 ohms.

Now suppose the lamp left for a few seconds unattended, while the carbons
are burning away and the arc is lengthening; in a very few moments the
resistance will have increased, owing to the greater distance between the
carbons, and we will suppose it to have become 5 ohms instead of 4.5.

The current passing will now be 45 / 5 = 9 ampères only.

In other words, a very slight lengthening of the arc has reduced the
current, and therefore the light, by 10 per cent.

Not only so, but 45 volts being needed to maintain an arc of {48} normal
length, it is insufficient to maintain a longer one, and in practice the
effect of leaving an arc under these conditions to itself for even a few
seconds is that it _goes out_, to the annoyance of the lecturer and the
confusion of the operator.

It is just _possible_ to work an arc lamp with a total E.M.F. of 45 volts
by giving one's whole attention to it and never taking the hand off the
feeding handle; but in practice no one with any experience would attempt
it. The arc would almost certainly go out several times during the
exhibition.

Now, take an example of a similar arc lamp consuming 10 ampères but worked
from a supply of 200 volts.

Our equation C = E / R must then obviously become

  C (10 ampères) = E (200 volts) / Total Resistance (20 ohms).

The resistance of the arc itself being the same as before, viz. 4.5 ohms,
it is obviously necessary to put an _extra_ fixed resistance equal to 15.5
ohms in series with it in order to make up the total of 20 ohms.

_Now_ leave the arc unattended until the resistance of 4.5 ohms has again
become 5 ohms; the only effect is that our current, instead of remaining at
10 ampères, has become 200 / 20.5 or 9.8 nearly, a difference which is
imperceptible.

This is not all, for it is an elementary rule in electrical science that
the total E.M.F. of any circuit distributes itself along that circuit in
proportion to the distribution of resistance.

In other words, our original E.M.F. of 200 volts will so distribute itself
as to reserve, so to speak, an E.M.F. of 45 volts for the arc, while the
resistance of this remains at 4.5 ohms, but directly this resistance
increases, the E.M.F. at the arc lamp terminals automatically rises, and
therefore the actual diminution in current is even less than the figures
above quoted. {49}

Should the arc tend to 'break' or go out, the resistance across it
automatically becomes infinite and the _whole_ 200 volts is at that moment
available to prevent the occurrence.

Under these conditions, therefore, the operator can safely leave the arc
for many minutes at a time. In carrying out experimental work I have often
left the lantern, walked up to the screen, discussed results with a friend,
and walked back, and the arc has shown no signs of misbehaviour whatever.

[Illustration: Fig. 28.--Resistance.]

In practice any current from 100 volts to 250 volts may be considered as
satisfactory for lantern work with a suitable resistance. Less than this
involves feeding the arc rather frequently, and more may give a nasty
shock, should the operator inadvertently touch a live wire, though I have
worked an arc lamp on a current of as much as 500 volts.

The _resistance_ usually consists of a suitable length of wire of high
resistance (Iron, German Silver, or those alloys known as Platinoid,
Eureka, Manganin, Beacon, &c., are most commonly used) wound in spirals on
a frame, and is generally supplied adjustable (Fig. 28), so that more or
less current may be used as desired. These resistances get pretty hot in
use, and care must be taken that they are placed where they cannot scorch
woodwork, &c., and in cases where the lantern is a fixture it is a good
plan to have the resistance bolted up against a wall once and for all. The
resistance may be placed anywhere in the circuit, so long as the current
passes through it, then through the arc lamp (or _vice versâ_), and back to
the other {50} pole of the supply main; it does not matter in the least
whereabouts it comes.

In cases, however, where one pole of the supply main is _earthed_, it is a
good thing to place the resistance in the 'live' side, as this keeps the
arc lamp within 45 volts of earth potential while it is working, to the
comfort of the operator should he touch a terminal or wire, though with an
ordinary lighting main there is no real fear of a dangerous shock in any
case.

The _amount of current required_ depends of course on the size of the
sheet, length of the hall, and density or otherwise of the slides; but it
is usually accepted in practice that the efficient light from a continuous
current arc lamp equals 100 candles per ampère, and therefore a 10-ampère
arc will give 1000 candles. This is sufficient for all ordinary halls and
slides, but where these latter are very dense, as for example with the
Lumière three-colour process, as much as 20 or 25 ampères may be required.

In these cases some special precautions must be taken for keeping the
slides cool, or the result may be disastrous, but this is a question that
will be referred to in a later chapter. A current of 10 ampères is pretty
safe for all ordinary slides, and may be taken as the normal current used
in large halls, though in arranging for the wiring it is as well to
stipulate for at least 12 or even 15 ampères, especially as there must
necessarily be a momentary increase of current at the instant the arc is
'struck.'

VARIETIES OF HAND-FED ARC LAMPS.--The pattern of hand-fed arc lamp
illustrated in Fig. 26 is only typical of many of the same general design,
and there are others in which the design itself is fundamentally different.
Of these the 'Scissors' arc lamp made by several firms deserves mention on
account of its simplicity and cheapness. As its name implies, the mechanism
resembles a pair of scissors, the carbons being attached to the ends of a
pair of levers hinged together {51} (Fig. 29). In this lamp centring
movements are usually dispensed with, the arc being clamped on to a tray
pin as in the case of a limelight jet. This is not, of course, so
convenient, and a further disadvantage of this pattern arc lamp is that the
feeding process gradually alters the position and angle of the carbons. In
fact, the one great merit of the lamp is cheapness, and where expense is an
object, it should certainly be considered.

[Illustration: FIG. 29.--'Scissors' Arc Lamp.]

Yet another arc lamp deserving of mention is the 'Parallel,' a name again
very aptly chosen, as the two carbons are either exactly parallel to each
other or very slightly inclined. In the former case the arc has to be
'struck' by touching the ends of the carbon rods with a piece of metal or
carbon. Of the actual manipulation of this lamp I have had very little
practical experience, but I have heard it well spoken of, though I believe
it has so far only been made for currents of 5 ampères or so.

Yet another type which must not be ignored is the 'Right-angled' pattern
(Fig. 30), a name again self-descriptive. The horizontal carbon is the
positive, and the vertical the {52} negative, and this lamp again is made
by several manufacturers in slightly different forms.

This pattern lamp is in my experience the best of all for _small_ currents,
say, of 5 ampères or so, but inferior to Fig. 26 for currents of 10 ampères
or more. This last remark perhaps hardly applies to _alternating_ currents,
which, however, I have not yet discussed. I cannot conclude this brief
category of arc lamps without referring to the _enclosed_ pattern, of which
the 'Westminster' is perhaps the best-known and most popular (Fig. 31).

[Illustration: FIG. 30.--'Right-angled' Arc Lamp.]

This is a lamp of the right-angled type, but the arc burns in a cylindrical
glass chamber, not air-tight, but partially so. After burning a few minutes
the oxygen in this chamber becomes used up and its place is taken by
carbonic-acid gas and other products of combustion, after which the carbons
burn away very much more slowly, and therefore require feeding at much
greater intervals.

This lamp again is chiefly made for small currents not exceeding 5 ampères
(and can therefore be used from any ordinary lamp socket), and for a
moderate-sized hall is on the {53} whole as cheap, efficient and simple a
lamp as any I am acquainted with. It can be supplied with or without
mechanical centring movements as required, and is usually sent out with its
own resistance for the particular current on which it is to be used, so
that it only requires connecting up to the nearest lamp socket, and is
ready for use.

[Illustration: FIG. 31.--'Westminster' Arc Lamp.]

It is _not_ sufficient for anything larger than a 12-foot sheet or for
working at a greater distance than, say, 40 feet, but within these limits
the lamp, and in fact _any_ good 5-ampère arc lamp, will be found quite
satisfactory and saves the expense of putting in a special cable.

AUTOMATIC ARC LAMPS.--Arc lamps for lantern work in which the feeding is
done automatically are also made. Like hand-fed lamps, they vary in exact
design, but all, or practically all, are so designed that the carbons are
brought together by means of springs or weights, and some form of 'brake'
controlled by a system of electro-magnets checks the {54} movement. As the
carbons burn away the arc lengthens, the current weakens, the
electro-magnets lose their grip, and the carbons move together until the
increasing current puts on the brake again. Some of these lamps are
'semi-automatic' only, that is to say, the arc has to be struck by hand,
while others perform this operation automatically as well, usually by an
additional magnet which draws back the carbons by the correct amount after
the arc is struck.

My frank advice to intending lanternists is to leave these lamps alone.
Some of them are satisfactory up to a point, but they are all apt to be
'jumpy,' and on the whole the hand-fed type is in my opinion to be
preferred.

ARC LAMPS ON ALTERNATING CURRENTS.--The alternating current is not so good
as the continuous for lantern work with arc lamps: the light per ampère is
not so great, the light has an irritating habit of travelling round the
carbons and there is always a slight 'hum.'

The sum total of these drawbacks is nothing very serious, provided that
proper arrangements are adopted, and I have frequently manipulated arc
lamps on alternating circuits with such good results that professional
lecturers have at first refused to believe that the circuit really _was_
alternating.

As it is frequently stated that to obtain a steady light with an
alternating current is impossible, I can understand their surprise, and I
can also understand the statement in question, as the problem is usually
tackled on entirely wrong lines.

It is almost always stated that arc lamps for alternating currents should
be arranged with the carbons _vertical_, and many makers actually so
construct their lamps as to allow of this.

To obtain a steady light under these conditions _is_ impossible and I pity
anyone who attempts it; but the statement that this is the best method of
working has been repeated so often that it seems to have been taken for
granted.

The best arrangement (in my hands at any rate) is to {55} slant the carbons
as for the continuous current, and also to have the upper carbon cored and
the lower one solid, but to use a rather larger lower carbon than would be
correct if the main were continuous.

Also the upper carbon should not be _quite_ so far back as with D.C.; to
have the front edges of the two carbons practically in line is about
correct, but the _exact_ position should be carefully adjusted to obtain
the steadiest light, and it will be found that a slight alteration makes a
considerable difference.

It is also a great help to have a weak electro-magnet, or its equivalent,
so arranged that it tends by its influence to keep the arc to the front. On
some lamps this is provided for, as even with a continuous current it is
quite harmless and, if anything, beneficial; but, if not, any competent
mechanic can easily fit an 'Induction Ring,' consisting of a single turn of
stout copper wire, which has sufficient magnetic influence to do all that
is required (Fig. 32).

This ring must be wired in series with the arc itself, and as the current
passing in it automatically reverses in synchronism with the arc, its
effect is _always_ to deflect the arc in the same direction, and care must
of course be taken that it is so wired that the deflection is forward and
not backward. This is the exact arrangement I have myself adopted, and I
never experience any difficulty on the score of the arc wandering.

Right-angled arc lamps, as described on pages 52 and 53, are also very
efficient on A.C. mains, and frequently these lamps are already equipped
with electro-magnets for the purpose required. The 'hum' of an alternating
current cannot be altogether eliminated, but can be reduced to a minimum
_by reducing the voltage as far as possible_.

As has been already said, the A.C. lends itself readily to transformation
of voltage, and I find in practice 90-100 to {56} be ideal. More than this
is inclined to be noisy, and less is apt to result in an unsteady arc.

The arrangement, therefore, which I recommend from long experience is to
employ a transformer to reduce the E.M.F. to 100 volts or thereabouts, and
then work with a resistance in the usual way (if the original current is
100 volts, of course _no_ transformer is required) with a properly
constructed arc lamp fitted with an induction ring or electro-magnet. No
difficulty should then be experienced in obtaining a good, steady, and
fairly quiet light.

[Illustration: FIG. 32.--Arc Lamp with Induction Ring.]

Any little 'hum' remaining can be silenced to a very considerable extent by
placing the entire lantern on a thick block of saddlers' felt, but in
practice I have never found this necessary with ordinary currents, though a
few abnormal circuits where the 'periodicity' is very high are noisier than
others.

[Illustration: FIG. 33.--THE OPTICAL SYSTEM OF A LANTERN.]

{57}

The following table gives the sizes and particulars of carbons for various
currents that I have found best in actual practice:

                    CONTINUOUS CURRENT

  Ampères.        + Carbon _Cored_.     - Carbon _Solid_.

    7-10                   12 mm.                   7 mm.
   10-15                   13 ,,                    8 ,,
   15-20                   16 ,,                   10 ,,

                    ALTERNATING CURRENT

  Ampères.    Upper Carbon _Cored_.    Lower Carbon _Solid_.

    7-10                   12 mm.                  10 mm.
   10-15                   13 ,,                   11 ,,
   15-20                   16 ,,                   13 ,,

       *       *       *       *       *


CHAPTER VII

THE OPTICAL SYSTEM OF A LANTERN

[Illustration: FIG. 33A.--Optical System of Lantern.]

As previously noted, the essential parts of an Optical Lantern are, in
order from rear to front: (1) The illuminant; (2) the condenser; (3) the
slide and slide stage; (4) the objective, to which must be added, (5) the
body or framework which holds the whole together. Fig. 33 is a diagrammatic
representation of the entire optical system and Fig. 33A shows all the
various parts _in situ_: A being the illuminant, shown in Fig. 33 {58} as
an arc lamp, B the condenser, C the slide stage, and D the objective. The
foundation, so to speak, of the whole instrument is of course the slide,
which, as made in this country, consists of a square of glass 3¼ inches
diameter, the slide itself being somewhat less than this on account of the
binding, &c.; in making calculations it is usually taken as a 3-inch
circle. Slides are usually made by binding together with strips of paper or
cloth two such squares, on one of which is the photographic film or
painting forming the picture, the other being simply a plain cover glass
placed over the slide surface to protect it, and between the two being
placed a paper mask with an aperture of whatever size or shape is required,
that of the aforesaid 3-inch circle being usually taken as the standard or
normal dimension for this aperture.

The slide being illuminated by one of the various methods discussed in the
previous chapters, is focussed on the screen by the objective, which must
be selected according to the size of picture required and the distance
between lantern and screen.

These points will be gone into later, and also details as to various types
of objectives and their respective advantages; but it may be said here that
a lantern objective consists usually of a combination of lenses of 2 inches
or 2½ inches diameter mounted in a rackwork focussing system at a distance
from the slide of 6 inches to 18 inches, according to the length of its
'focus.' As our slide is from 3 to 3¼ inches diameter, it is evident that
all the light radiating from this cannot possibly get through the objective
unless it is _converged_ upon it, and to do this is the function of the
condenser. The following two diagrams, Figs. 34 and 35, will make the
matter clear.

S represents our glass slide of 3 inches clear diameter, R the radiant or
illuminant, and L our objective, shown here for the sake of simplicity as a
single lens.

The slide is well illuminated by the light emanating from {59} R, but it is
obvious that the bulk of this light will never pass through the lens, and,
in fact, only the very centre of the slide will under these circumstances
appear upon the screen at all.

[Illustration: FIG. 34.--Optical System without Condenser.]

[Illustration: FIG. 35.--Action of Condenser.]

What is evidently wanted is to _converge_ these outer rays, or in other
words to bend them in so that they also pass through the objective, and
this is the function of the condenser as illustrated in Fig. 35. The
condenser is here represented also by a single lens, but in practice it
also is invariably constructed of two or even three lenses, for both
optical and mechanical reasons. It is evident from the above diagrams that
the condenser must be somewhat larger in diameter than the slide itself,
and condensers for ordinary lantern work are usually 4 inches to 4½ inches
diameter. The former size {60} will suffice if the condenser is placed very
close to the slide, but it is often advisable to leave a little intervening
space, especially if the illuminant is a powerful one, in order to allow
any condensation of moisture readily to evaporate and escape. Hence
lanterns for long range work (which involve, of course, good illumination)
are usually made with condensers of 4½ inches diameter. Lantern condensers
of to-day usually take one of the two forms shown in Fig. 36, but the exact
curve must be left to the manufacturer, as the focus of the condenser must
have a definite relation to that of the objective. Taking, however, the
design of E, the most common of all, the two lenses should not be exactly
similar unless the objective is pretty short in focus, or, in other words,
unless the distance of the illuminant on the one hand and that of the
objective on the other are approximately equal. If the lantern is intended
for long range work, that is equipped with a long focus objective, the
front component of the condenser should also be constructed longer in focus
(that is to say, with a shallower curve) than the rear one, and it is
amazing how careless manufacturers are in this respect. If, as is often the
case, the lantern is fitted with several objectives of different foci, it
is usually necessary to supply alternative condensers also, or at least to
supply an interchangeable front component.

[Illustration: FIG. 36.--Forms of Condensers.]

If the entire condenser is too long in focus, light is lost; if too short,
it is impossible to obtain an even disc, as there is invariably a dark
patch either in the centre or round the edges.

The mounting of the condenser also varies with different makers; but it
must be remembered in any case that it gets {61} extremely hot, especially
the back component, and hence the glass must be mounted _loose_ in its
cell, otherwise there is great danger of it cracking. Also the space
between the components should be well ventilated, in order to provide for
the escape of moisture which usually at the start of a lantern exhibition
is deposited upon the glass, and should be got rid of before the actual
lecture commences.

Even with all care, the back component of a condenser will sometimes crack,
though such an accident should be a rare occurrence; and hence a
professional operator will usually provide himself with a spare lens, and
the condenser should be so constructed that it can readily be changed, and
with as little delay as possible.

Condenser lenses as made in this country are usually ground from the glass
known as 'English Crown,' and comparatively rarely crack; but they are very
slightly green in colour. French condensers, on the other hand, are whiter,
but the glass is more brittle, and a fracture a more common occurrence. The
French variety are (or were before the war) cheaper and generally met with
in cheaper instruments. More expensive lanterns are usually fitted with
English condensers, as the tinge of green is almost imperceptible, and the
advantage as regards greater security pretty considerable.

THE SLIDE CARRIER AND SLIDE STAGE.--Taking still the optical system of the
lantern in order from back to front, we now come to the slide, slide
carrier, and slide stage. The slide itself has already been described, and
the carrier is simply a mechanical contrivance, usually of wood, designed
for the purpose of readily changing the pictures and which in its turn fits
into the stage of the lantern. It may be asked why, if slides are now
always made to a standard size, the slide carrier should not itself be
built into the lantern and form the stage; but the answer is, in the first
place, that slides of a different size, _i.e._ American or Continental,
_may_ be met with, {62} and also that there are various mechanical slides
on the market--for example, chromotropes or scientific models, such for
instance as are made to illustrate the movements of the planetary
bodies--and these slides are permanently mounted in wooden frames which
could not be put into a carrier. The commonest form of carrier is that
known as the 'Double Sliding' pattern (Fig. 37), which consists of a frame
with two apertures for the slide, and an outer frame through which this
itself slides and which fits the stage of the lantern.

[Illustration: FIG. 37.--Double Sliding Carrier.]

This carrier, as will be seen, allows the next picture to be placed in
position in the second aperture while the former one is being projected,
and at a signal from the lecturer, the inner frame slides smoothly through
the outer, and the slides are thereby changed. This carrier is simple,
cheap, and quiet in its action; its one disadvantage is that each alternate
slide has to be inserted from opposite sides of the lantern, and unless the
operator is fairly tall this almost necessitates an assistant.
Nevertheless, the carrier is the most popular of any, its other advantages,
especially as regards price, being so great. It is usually constructed in
such a way that the slide, as it moves out from the central position,
automatically rises in its groove in order to facilitate quick removal.

Another pattern deservedly popular is that known as {63} 'Beard's
Dissolving Carrier' and is shown in Fig. 38. In this ingenious carrier all
the slides are inserted from the same side, the oncoming slide being pushed
_in front_ of its predecessor, and being therefore somewhat out of focus it
produces a blur on the screen.

The movement is performed by pushing in a projecting handle, and on
withdrawing this the slide which is finished with comes with it, and the
finish of the movement presses the new slide back until it is in its proper
position and in focus.

[Illustration: FIG. 38.--Beard's Dissolving Carrier.]

The entire action is simpler than it sounds, and the temporary blurring of
the image on the screen during the process of changing is supposed to give
somewhat the effect of 'Dissolving Views,' and hence the name 'Dissolving
Carrier.'

This appliance is three times the price of the 'Double Sliding' pattern,
but the fact that it is worked from one side only is a decided advantage,
though on the other hand it is not (unless great care is used) quite so
silent in its action as the 'Double Sliding' type.

A further modification of this carrier adapts it to take any of the
recognised 'foreign' sizes of slides, so that if a few American ones, for
instance, are met with among a collection of English manufacture, there is
no need to change the carrier. {64}

There are other varieties of carriers on the market which there is no need
particularly to describe, such as, for example, carriers fitted with roller
curtains to give the effect of a curtain rolling up, magazine carriers to
hold twenty-four or more slides and exhibit them in rotation, and other
patterns too numerous to mention. Of these the reader must be left to judge
for himself, but, generally speaking, _simplicity_ in a carrier is the most
important point to be looked for, and complications, however ingenious,
should be avoided.

[Illustration: FIG. 39.--Focussing Action of Lens.]

The lantern stage must also receive consideration, but it will be better to
discuss it as part of the mechanical construction of the lantern.

THE OBJECTIVE is really the most vital part of a lantern, as the definition
of the picture almost entirely depends upon the excellence or otherwise of
this lens. This will be obvious at once when it is realised that the
objective has to project on to the distant screen a greatly magnified image
of the comparatively small lantern slide, and the intending purchaser is
strongly advised to economise almost anywhere rather than on this item.

The action of a lens in focussing the image is perhaps best explained by a
simple diagram (Fig. 39), from which it will be seen that all the rays
proceeding from any one point on the object are re-converged (when the lens
is in focus) to a definite point on the image, and the perfection of the
picture depends upon the lens performing this function accurately. {65}

The imperfections are chiefly two, viz. those known as chromatic and
spherical aberration respectively. Chromatic aberration simply means that
all the colours composing the original beam of, say, white light are not
equally refracted or converged, and therefore do not meet again at the same
spot (the well-known prism or lustre effect), and reveals itself by
coloured fringes round the edges of the various details in the picture.

[Illustration: FIG. 40.--Achromatic Lens.]

By spherical aberration we mean that the light falling upon the centre of a
lens is not brought to a focus at exactly the same spot as the marginal
rays, and a general want of definition is the result, usually accompanied
also by a want of 'flatness' in the image, that is to say the edges of the
picture do not focus at the same time as the centre.

Chromatic aberration is easily cured by using an achromatic or compound
lens made by cementing together two lenses of crown and flint glass
respectively, as in Fig. 40.

It will be seen that the flint glass component by itself is a _concave_
lens and therefore neutralises in part, or in whole, the convex crown lens.
Flint glass has both greater dispersive power and also greater refractive
power than crown glass, but fortunately not to the same _degree_; hence a
compound lens made in this way and with curves carefully worked out may
have its chromatic effect entirely neutralised while retaining very
considerable refractive or 'focussing' power, and simple achromatic
objectives of this type are quite inexpensive.

In lanterns intended for Science demonstration, as distinct from the mere
projection of slides, lenses of this pattern are very frequently used, as
they will project the latter when required reasonably well, and for the
demonstration of {66} experiments or of apparatus on the screen have
advantages that need not be discussed here.

For very long focus lenses also they are sometimes employed, as the trouble
from spherical aberration is much less apparent with lenses of long focus
than with short, and the difference in expense is much more in the former
case than in the latter. For short focus lenses, however, as used in
moderate-sized halls, they are not good enough, and the type of lens almost
universally employed is that known as the 'Petzval' combination (Fig. 41).

[Illustration: FIG. 41.--Petzval Combination.]

This lens really consists of two achromatic combinations, the pair at the
front being cemented together, and that at the rear having an air space
between. The combination is so designed that the spherical aberration of
the one pair neutralises that of the other, and the result is or should be
a lens corrected both for chromatic and spherical aberration.

These lenses, however, vary very much in the perfection of their results,
and as they are at present usually imported in bulk from France, the
customer does well to insist upon a demonstration of his own particular
lantern before acceptance.

The magnifying power of a lens depends upon its 'focus' multiplied by its
distance from the screen, and the focus in the case of a simple lens is
easily determined by the familiar 'burning-glass' experiment, that is by
focussing an image of the sun upon a piece of paper and measuring
accurately {67} the distance the lens must be away to produce the most
concentrated spot.

In practice it is sufficiently accurate to focus a distant window, or other
luminous object, upon the paper, any error obtained by this method being
for ordinary purposes a negligible one.

With a compound lens, such as a 'Petzval' combination, this method does not
hold good, as the optical centre of such a lens is not necessarily midway
between its two components.

The actual focus can be got pretty approximately by focussing a window or
other object as before and measuring the distance from one definite point
(say the front edge of one of the lens cells) to the paper, then turning it
round and taking a second measurement from the _same_ point, the mean
between the two measurements giving the actual focus.

In practice the 'simple equivalent focus,' as it is termed, of a lantern
lens is usually determined by measuring the magnification of the image
thrown upon the screen, when, by knowing the original size of the slide (a
'standard' slide of 3 inches diameter is usually taken) and the distance
between lantern and screen, we get the focus from the following very simple
equation:

  Diameter of picture on screen    Distance between lens and screen
           (in feet)                        (in feet)
  ----------------------------- = ---------------------------------
  Diameter of slide (in inches)      Focus of lens (in inches)

or perhaps more simply still:

  Distance between lens  × Diameter of slide
  and screen (in feet)      (in inches)
  ------------------------------------------  = Focus of lens in inches;
          Diameter of picture (in feet)

or, if we know the focus of the lens but want to know how far from the
screen we must go to produce a given-sized picture, the formula will be:
{68}

  Diameter of picture  × Focus of lens
        (in feet)         (in inches)
  ------------------------------------ = Distance required (in feet).
      Diameter of slide (in inches)

It is handy for the lanternist to remember that, dealing with a standard
3-inch slide, a 6-inch lens will _always_ give a picture whose diameter is
_one-half_ the distance from lens to screen, a 12-inch lens half this again
or _one-quarter_, and a 9-inch lens half-way between the two.

Bearing these simple figures in mind, the approximate distance can usually
be _guessed_ sufficiently near for the first trial, and then the lantern
shifted a little nearer or the reverse as required.

The following table may, however, be useful, as showing readily the
magnification produced at different distances by lenses of given foci:

  +------+---------+--------+--------+--------+--------+--------+--------+
  | Disc |  Focus  |  Focus |  Focus |  Focus |  Focus |  Focus |  Focus |
  |wanted| 4½ in.  |  6 in. |  8 in. |  10 in.|  12 in.|  15 in.|  18 in.|
  +------+---------+--------+--------+--------+--------+--------+--------+
  | feet.| ft. in. | ft. in.| ft. in.| ft. in.| ft. in.| ft. in.| ft. in.|
  |   9  | 13   6  | 18   0 | 24   0 | 30   0 | 36   0 | 45   0 | 54   0 |
  |  12  | 18   0  | 24   0 | 32   0 | 40   0 | 48   0 | 60   0 | 72   0 |
  |  15  | 22   6  | 30   0 | 40   0 | 50   0 | 60   0 | 75   0 | 90   0 |
  |  18  | 27   0  | 36   0 | 48   0 | 60   0 | 72   0 | 90   0 |108   0 |
  |  20  | 30   0  | 40   0 | 53   4 | 66   8 | 80   0 |100   0 |120   0 |
  |  25  | 37   6  | 50   0 | 66   8 | 83   4 |100   0 |125   0 |150   0 |
  |  30  | 45   0  | 60   0 | 80   0 |100   0 |120   0 |150   0 |180   0 |
  +------+---------+--------+--------+--------+--------+--------+--------+

THE DIAMETER OF THE OBJECTIVE.--The diameter of the objective must depend
to a certain extent upon its focus in the case of a double combination such
as a Petzval. These lenses consist, as has already been said, of two
achromatic components some distance apart, and for technical
considerations, which need not be discussed here, the _distance_ between
these components is usually about two-thirds of the focal length. This is
not a universal rule, as the lenses of different makers vary a good deal;
but it is generally a fact {69} that the longer the focus of the lens the
greater is usually the separation between the two lens systems.

The entire lens therefore mounted in its tube resembles a _tunnel_ of
varying length according to its focus, and through this tunnel a _cone_ of
light rays have to be passed. It is plain, therefore, that a lens of long
focus, which in practice means a long tube length, must be made also of
large diameter, or a portion of the cone will be cut off, with a consequent
loss of light.

In practice lenses up to 6 inches focus are usually made of 2 inches
diameter, and there is no advantage in a larger size. With a lens of 8
inches focus there is a slight gain in increasing the diameter to 2-3/8
(the next 'standard' size), and lenses of longer focus than this should
certainly be 2-3/8 inches up to, say, 12 inches focus, when a lens of 3
inches diameter is preferable. These large lenses are, however, very
expensive, both in themselves and also on account of the fact that their
weight entails heavy and expensive brass mounting, and hence lenses up to
14 or 15 inches focus are often supplied in the 2-3/8 size for reasons of
economy.

To sum up, _short-range_ lanterns, as they are called, are usually fitted
with lenses of 2 inches diameter, and _long-range_ instruments either with
3-inch lenses or the intermediate size of 2-3/8 inches. If a lantern is
purchased for either long or short-range work, it is usually fitted with a
brass front for a large lens, and so arranged that a shorter focus lens of
2 inches diameter can easily be interchanged, utilising the same brass
mounting.

Lenses of _variable_ focus have also been designed, in which an additional
lens can be added or subtracted to increase or decrease the focal length;
but nothing very practical has yet been achieved in this direction, and
therefore these 'Omnifocal' lenses have never come into general favour.

Objectives like condensers want cleaning at times, and care must be taken
not to scratch the glass, as the concave lens of each component is of flint
glass, and very soft. A {70} clean chamois leather is the best thing to
use, but a soft cloth, or even a handkerchief, may be employed with care.
It is very important that a lens be reassembled, after cleaning, the
correct way, as a single lens reversed would utterly spoil the definition.
The front component is usually balsamed together, and therefore all that is
needed is to see that the whole combination is not reversed. In the Petzval
system this lens should have its convex constituent towards the screen
(Fig. 41). The back combination is usually loose, and the two lenses are
sometimes separated by a thin brass ring. In the Petzval lens the concave
element should be inside, with its concave surface outwards, the deep curve
of the other lens should fit into this concavity, and the flatter curve
face towards the condenser. One or two makers, however, have introduced a
modification of the Petzval system in which the whole of this back
combination is reversed, and the exact arrangement should therefore be
noted very carefully when taking the lens to pieces.

       *       *       *       *       *


CHAPTER VIII

THE BODY OF THE LANTERN

We now come to the mechanical construction of the optical lantern, and a
great variety of design presents itself, according to price, type (_i.e._
short range or long range), and the individual ideas of the various makers.

Lantern bodies as a rule are now made of metal, although up till quite
recently the better class instruments were more usually made of polished
mahogany lined internally with iron; but there has of late been a consensus
of opinion in favour of metal only.

In the cheaper lanterns this metal body is usually made either of Russian
iron or of sheet-iron tinned and japanned, {71} there being little to
choose either in price or quality between the two varieties, and in all but
the very cheapest instruments the front is usually of brass.

In better lanterns the body is more often made of enamelled steel, the
front as before being of brass; but brass, copper, or aluminium are also
used occasionally for the body of the lantern.

[Illustration: FIG. 42.--Hughes' Short-Range Lantern.]

In deciding upon the type of body to be purchased the main considerations
to be borne in mind are: (1) The type or types of illuminant to be used, a
powerful arc lamp for example requiring a larger body than is necessary for
a weaker radiant; (2) the size and position of the lens to be carried, a
Petzval objective of say 3 inches diameter which has to be supported at the
end of a long brass mount for long-range work obviously demanding a body of
greater strength and rigidity than is required with a 6-inch focus lens of
2 inches diameter; (3) price.

Fig. 42 shows an extremely good lantern body for short-range work made by
Messrs. Hughes, the illustration depicting the instrument complete with a
'Luna' methylated spirit lamp, though, of course, any other illuminant
suitable for a small lantern could be used instead. {72}

This lantern illustrates well one point that has already been emphasised as
important, viz. the ventilation of the condenser. It will be noticed that
this is placed _outside_ the body of the instrument instead of inside as is
usual with larger bodies, and that wide slots are cut in the condenser
mount to allow free escape of steam.

Other points of this excellent design are the screw adjustment to the slide
stage (facilitating the use of special slides, such, for example, as those
illustrating the movements of the planetary bodies which sometimes involve
the use of extra thick frames) and a simple but efficient tilting
arrangement to the base.

[Illustration: FIG. 43.--Long-Range Lantern.]

Such a lantern is hardly suitable for a powerful arc lamp or limelight jet,
or for heavy long-range lenses, but is a very good typical instrument for
use in moderate-sized halls, and a lantern of this general type is usually
found in lantern catalogues, though, of course, the exact designs vary
according to the ideas of the manufacturer. Of lanterns for long-range work
a good example is perhaps Messrs. Newton & Co.'s 'Intermediate' pattern
(Fig. 43).

This again is only typical of many others by the various makers, but the
principal points are common to all. These are: (1) The large and
well-ventilated body; (2) the long {73} baseboard; (3) the strong and
massive brass front necessary to carry the large long-range lenses; (4) the
velvet curtain at the back to close in any stray light from a powerful arc
lamp.

[Illustration: FIG. 44.--Connections for a Bi-unial Lantern.]

The two foregoing designs are perhaps sufficiently typical of lantern
bodies in general to make further detailed description of individual
designs unnecessary; but reference should be made to features which special
requirements may render advisable.

Under this heading mention must be made of _Bi-unials_ or Double Lanterns,
as used for the once famous 'Dissolving Views.'

A bi-unial lantern consists essentially of two different instruments, each
complete with its limelight jet or other illuminant--front, condensers,
objective, &c., usually mounted on one body--and with some arrangement for
'dissolving' or turning the light in each lantern gradually on and off.

Fig. 44 shows the back view of such a lantern with two limelight jets and
dissolving tap, this piece of mechanism (shown below in the illustration)
being so arranged that when the lever is horizontal _both_ lanterns are on
full, but moving the lever either way cuts off the gas supply to one
lantern. In the case of limelight the tap should always operate by cutting
off the oxygen supply in advance of the coal gas (in order to avoid a
'snap'), and the latter should never be cut off entirely, but a small bead
of flame left to keep the jet alight, until the lantern is required for the
next slide. {74}

This is usually arranged for by means of a bye-pass, and a bye-pass is
sometimes provided on the oxygen side as well, but is usually discarded in
practice.

A bi-unial lantern can be worked in the same way with acetylene gas, but
with the electric arc it is impossible to turn the light on and off
gradually, and in practice dissolving must be done by keeping both lanterns
fully alight, and using a dissolving shutter, that is a movable shutter
that covers each objective alternately. The same arrangement must be used
with other illuminants, such as oil, only in this case the lanterns must be
mounted side by side, on account of the tall chimneys. With oil lamps the
arrangement answers fairly well, the dissolving fan, as it is termed, being
made with serrated edges which give the _gradual_ obliteration required;
but with the electric arc the extremely sharp definition becomes a serious
difficulty, and a good dissolver for this illuminant has never yet been
found, though, in view of the fact that dissolving views are more or less a
thing of the past, the matter cannot be regarded as important.

The advantages claimed for a double lantern are two: first, a 'Dissolving'
effect by which one picture fades gradually into the next, and which is
supposed to be more pleasing than the movement of a carrier; and second,
'Dissolving Effects' can be shown, such as exhibiting a landscape by day
and changing it into a moonlight scene, or bringing on the appearance of a
snowstorm, which can easily be done by means of a roller slide, with minute
perforations shown in motion by the second lantern while the landscape
remains on the screen from the first. In the days when dissolving views
were all the vogue, a third or even a fourth lantern has been added for
more complicated effects, and at the famous Polytechnic demonstrations of
years ago, I believe that as many as six were sometimes employed.

In these days of the cinematograph it is doubtful how far interest in such
effects could be revived, and a lantern has {75} gradually come to be
looked on more as an instrument for showing illustrations as required by
the lecturer rather than as a pleasing exhibition in itself, and as
dissolving views have lost their attraction, the double or triple lantern
has been relegated to the limbo of antiquity.

[Illustration: FIG. 45.--Beard's Circulating Water Tank.]

Among other 'special' lanterns should be mentioned models made with
water-cooled stages, for use with very delicate slides. This elaboration is
not necessary with ordinary slides and illuminants of moderate power, but
where very delicate slides, such as specimens of natural colour
photography, have to be shown, it is an advisable precaution to pass the
beam of light first through a tank of water in order to absorb the heat.
Lanterns intended for this work are usually constructed with a kind of
double stage, a glass trough of water fitting into the rear aperture and
the slide-carrier into the front one. Such an arrangement answers quite
well for most purposes, but for extreme cases lanterns are equipped with a
trough connected to a large outside tank and complete circulatory system,
after the manner of the cooling tank of a gas engine.

Such a lantern, constructed by Messrs. Beard, is illustrated in Fig. 45,
and it will be seen that in this instrument the water trough is placed
between the lenses of the condenser. {76} This is a very good position, as
the beam of light at this point is, or should be, parallel, whereas between
the condenser and the slide it is convergent, and therefore a condenser of
a larger diameter than the slide must be employed in the latter case if the
trough is of considerable width.

While dealing with 'Special' lantern bodies, we should perhaps just mention
here the numerous pattern lanterns made for the demonstration both of
lantern slides and of Scientific Phenomena, such as the projection of
insect life or other microscopic objects, polarised light experiments,
electrical apparatus, opaque objects, &c. A detailed description of these
lanterns and how to use them belongs to the second part of this work, as
also does the popular cinematograph; but educational institutes, and even
boys' clubs, when considering the purchase of a lantern, might well reflect
whether it would be advisable to spend a little more money in the
acquisition of an instrument which can be utilised for a variety of
purposes.

       *       *       *       *       *


CHAPTER IX

LANTERN BOXES, STANDS, READING LAMPS, ETC.

Having now discussed all the essential parts of a lantern, the next points
to be considered are those of lantern boxes and stands. It is best to take
these together, as more often than not a lantern is arranged to stand upon
its box during use, and the plan is both convenient and simple. The whole
question is one to be settled upon its own merits in each individual case.
Sometimes neither box nor stand is wanted at all. The lantern is put away
into a locked-up cupboard or other safe place, and used upon a permanent
support or (as is often the case in a church) from a gallery at the back.
{77}

In most cases, however, a box of some sort is desirable, and the two main
considerations are strength and simplicity.

All patent arrangements, such, for example, as those in which the sides of
the box fall down and provide trays for the slides, are beautiful in
theory, but cannot be recommended in practice. A good, simple and
substantial box is what is required, preferably painted black, and provided
with strong handles.

One addition may be permitted, viz. a tilting top. Some means for tilting
the lantern is always advisable, as it is seldom convenient to raise the
instrument to the level of the centre of the screen, and a slight upward
elevation does not appreciably distort the image. This arrangement for
tilting may be either embodied in the lantern itself, as for instance in
the instrument shown in Fig. 42, or may be provided for on the box or on
the stand, if a stand is used.

It is, perhaps, an elaboration that may be regarded as not strictly
necessary, as a book or two or other article may be placed under the
lantern base as required; but a tilting arrangement is so convenient that
it can be strongly recommended, and the addition is not expensive.

For large, long-range lanterns a strong deal box, on which the lantern can
stand, is usually all that it is desirable to purchase in the way of a
support. A good solid table can usually be found, which will do all the
rest, as it must be remembered that a slight tilt at a long range means a
good deal of total elevation.

Where this is not procurable a stand must be provided, and this for a large
lantern should be strong and rigid. Anything in the way of a collapsible
tripod should be avoided, but such an arrangement as Fig. 46 is quite good
and rigid enough for all practical purposes.

For a _small_ lantern a tripod stand is quite suitable, though care must be
taken that one of the legs does not get kicked, either by accident or
design, or the result may be a catastrophe. {78}

_Slide Boxes._--On this subject not much need be said. The variety of
patterns on the market is endless, some being designed from the point of
view of safe transit by post, others for convenience of storage and
classification. It is essentially a case where each individual user must
use his or her taste, and in any case the question of the box is one for
the owner of the slides rather than for the lanternist.

[Illustration: FIG. 46.--Quadruple Lantern Stand.]

READING-DESKS, LAMPS, AND SIGNALS.--Some form of reading lamp for the
lecturer is generally considered to be part of a lanternist's equipment,
and the most usual pattern is fitted with a candle, after the manner of a
carriage lamp, or else constructed to burn colza or other vegetable oil,
such as supplied for cycle lamps. Oil gives the brighter light, but is apt
to get spilled in transit, hence a candle lamp is the more {79} convenient
for a travelling lecturer, while oil is to be preferred if transport is not
a factor to be considered.

These lamps are usually constructed with a red flashing signal at the rear,
actuated by a simple lever, by which the lecturer can communicate his wish
for a change of slide, &c., to the lanternist (Fig. 47).

[Illustration: FIG. 47.--Reading Lamp.]

There are various other devices used for the same purpose, such as a
castanet, to be held in the lecturer's hand and clicked when necessary, an
electric bell to ring in the lantern box, &c. If this latter is used it is
usual to remove the gong, the buzz of the hammer being sufficiently loud
without it. Some lecturers again prefer to use no such apparatus at all,
but simply to say 'Next slide' as required, or to tap on the floor with a
pointer, and the choice of a suitable means of communication between
lecturer and lanternist must be largely a matter of individual selection.
More elaborate _reading-desks_ are also supplied by most makers, but here
again judgment must largely come into play in what is hardly a technical
matter.

       *       *       *       *       *


CHAPTER X

SCREENS AND SCREEN STANDS

The best of all screens for lantern purposes is undoubtedly a smooth
whitewashed wall, and this is now provided in many halls where lantern
exhibitions are usual. In places where this is not practicable the next
best substitute is a canvas {80} screen, which rolls up and down (Fig. 48).
This can be obtained from any good maker, but again can only really be used
as a _fixture_ in the hall where the lantern is to be used. It can,
however, be fitted into a wooden box which can be painted or varnished to
suit the other architecture, and the provision of such a screen is to be
strongly recommended whenever possible. If portability is required, a linen
or calico sheet that can be folded up is necessary, but this can never be
hung absolutely flat, and also loses a considerable amount of light by
transmission.

[Illustration: FIG. 48.--Roller Screen.]

A so-called 'transparent' sheet is made of very thin linen, and intended to
work with the lantern _behind_ it, showing the picture through the linen to
the audience on the other side, but this is seldom used except in the open
air for religious or political meetings, &c.

An _opaque_ sheet can be had in one piece up to 9 feet square; larger sizes
than this must have at least one seam, and most skilful sewing is
necessary, especially with large sheets consisting of several strips sewn
together.

Sheets such as these are usually supplied with either eyelet holes round
the edges or else linen tapes sewn on, and the exact method of hanging must
be left to circumstances. {81}

In the case of a small sheet it will be sufficient to stretch it at the
four corners, and this can often be done by screwing into the walls or some
convenient girder two screw eyes and similar eyes into the floor, all four
being considerably farther apart than the size of the sheet.

[Illustration: FIG. 49.--Portable Screen Stand.]

A stout cord being then passed through the two upper eyes, long enough for
both ends to reach near the floor, one end of each can be fastened to the
two top corners of the sheet and the latter drawn up, the two bottom
corners being afterwards stretched and tied down tightly to the lower eyes.
In the case of large sheets this hardly suffices, and it will be found
necessary to fasten the sheet at intervals all round or it will exhibit
awkward creases, and this again is a matter where the lanternist must use
his own initiative according to the possibilities.

In some halls the erection of a sheet in the way above described is a sheer
impossibility, and in such cases a frame must be made by nailing strips of
wood together, or better by utilising a portable screen stand (Fig. 49).

These stands are usually made of bamboo, with short brass connecting tubes,
and the method of using them is so obvious that a description need hardly
be given. The screen frames are supplied by all the leading opticians, but
an intending purchaser would be well advised to see one erected before
ordering. I have actually seen a 12-foot screen frame offered for sale that
was too weak to carry its own weight, let alone the weight of the sheet!

       *       *       *       *       *


{82}

CHAPTER XI

THE PRACTICAL MANIPULATION OF A LANTERN

Having now described the optical lantern in its various forms and the more
important accessories, we come to the question of practical manipulation.
In making arrangements for an exhibition the first thing to be seen to is
to ensure that every accessory that will be required will be there, and the
best plan is to make a complete list of all sundries to be provided. Such
items as string (for the sheet), lime tongs if limelight is used, pliers
for changing carbons if the arc is to be the illuminant, screw-driver,
matches, the _key of the lantern box_, and other similar items, are likely
to be left behind unless such a list is made and carefully checked. On
arriving at the hall, the first thing to be done as a rule is to get up the
sheet, after which the professional operator generally begins to feel happy
again.

The next thing, if it has not been done first, is to determine the position
of the lantern, and this, as has been explained in Chapter VII, is a matter
of the size of picture to be shown and the focus of the objective.

It is a mistake to show too large a picture; a little 'white' round the
edges is a good thing, and it is better to have a small disc well
illuminated than a large one less bright. Convenience, however, must also
be considered, and it is often justifiable to go back a few feet farther
than other considerations would dictate in order to place the lantern in a
gallery or other spot where it is out of the way.

Having fixed the position of the lantern, it should be got into place, the
cable or tubing connected or whatever else is necessary, according to the
illuminant to be used. It should then be lit up, the flasher of the lens
opened, and the light {83} centred sufficiently to produce some sort of
disc upon the screen. (It is, of course, presumed that the lenses, &c.,
have previously been cleaned.)

A carrier should now be placed in the stage and a slide inserted into it,
and the method of doing so requires a little explanation. The slide must be
placed in the carrier upside down, as will be obvious to anyone who has
studied Chapter VII, but in addition to this it must be turned the correct
way, otherwise the picture will be reversed from left to right. This in the
case of certain subjects, such as a copy of a picture, may not greatly
matter; but in slides depicting buildings or landscapes with which the
audience may be familiar, or worse still, printing or writing, is a serious
blunder.

Slides made by a commercial firm will usually be 'spotted,' that is to say,
will have two white spots on the face of the slide when the latter is
viewed in its correct position, and at the top. The slides should be turned
upside down and placed in the carrier with the spots, of course, now at the
bottom and _towards the condenser_.

If a slide is not spotted it should be viewed as it is to appear on the
screen, and then placed in the carrier with the face that was towards the
operator as he viewed it turned to the condenser, and of course inverted.

The above remarks apply only in cases where the image is thrown _on_ the
screen; in the comparatively rare instances where it is shown _through_ the
latter the slides must be turned round laterally, but of course still
inverted. The slide having been placed in the stage it should be 'focussed'
by racking the objective in or out, and if necessary pulling out the draw
tube as well until the image on the screen is sharply defined. So far the
light has only been roughly centred, sufficiently so to enable the slide to
be focussed, and to complete the operation both slide and carrier should
next be taken out of the lantern, leaving a clear disc on the screen, and
this disc may resemble any of the appearances shown in Fig. 50. {84}

If it resembles A the light must be moved to the left, if like B to the
right, like C it must be lowered, like D it must be raised, always moving
it to the side opposite to the dark shade until this is central on the
disc. If it now resembles E, the light must be moved nearer the condenser;
if, on the contrary, the centre is dark, it must be drawn back until
finally the circle should be as nearly as possible clear and bright all
over, as at F.

[Illustration: FIG. 50.--Adjustment of the Light.]

It is important to note that this adjustment _cannot_ be properly made
while a slide is in position, and neither can it be made until the lantern
has been focussed, so the above procedure is the only way to get a
satisfactory result. With some of the larger illuminants, such as a
paraffin-oil lamp, there are no centering adjustments, the size of the
radiant rendering exact centering unnecessary, and generally speaking the
smaller the luminous point, the more exact must the operation be.

In the case of such illuminants as acetylene or limelight {85} care must be
taken that they are turned fully on before centering, otherwise turning on
the fuller amount afterwards will raise the position of the luminous spot.

The centering achieved, the slide carrier may be replaced, the first slides
placed in position, the remainder arranged in their proper order, the
system of signalling with the lecturer determined, and all is ready.

If there is still an interval before commencing, the light may be switched
off or turned out, or in the case say of limelight, turned down very low
until wanted.

It is of extreme importance to see that all the slides are in their right
order, though the duty of seeing to this usually rests with the lecturer
rather than with the operator. I remember hearing of one lecture on the
life of Queen Victoria, when the lecturer announced, 'The next picture will
be a photograph of the Royal Prince who for many years shared the Throne
with our gracious Sovereign.' At the words the operator brought on the next
slide, which proved to be _a restored specimen of a prehistoric monster_
(tableaux!). Such mistakes 'bring down the house,' but in serious lectures,
and especially at religious services, cannot be too carefully guarded
against.

Mention has already been made of the liability of moisture to condense on
the surfaces of the condensers or slides, and to avoid this, so far as the
condensers are concerned, it is well to light up say ten minutes before the
lantern is actually wanted, or alternatively to take out the condensers and
thoroughly warm them in front of a stove, or to place them wrapped in a
cloth on hot-water pipes. The slides should in the same manner be warmed
before using and should be finally held above the lantern or placed on the
top, if this is flat, the last thing before being placed in the carrier. If
these precautions are omitted, on a cold night the first surface of the
condenser will become so covered with moisture as to almost obscure the
slide, and this will quickly disappear {86} with the heat of the lantern.
Next, the two inner surfaces of the condensers will behave in turn in the
same way, and will take considerably longer to clear, especially if the
ventilation of the condenser is poor; then the fourth surface will take up
the running, and finally, when the lanternist is congratulating himself
that the trouble is over, each successive slide will become affected in the
same way. With an operator who knows his business, none of these troubles
should occur.

ACCIDENTS.--These will occur sometimes, even in the best managed
exhibition; the rubber tubing feeding a limelight jet gets kinked or
trodden on, or a fuse melts if electric light is being used, &c., and out
goes the light. In such cases a loud request such as, 'Would you mind
turning up the light for a minute, please,' accompanied by a good-humoured
laugh, usually allays the fears of 'nervy' people. An operator must never
get 'nervy' himself. I have known of more than one fiasco because some
little hitch occurred, and two or three timid ladies crowded round and
asked anxious questions, till the lanternist lost his head. In one such
case the cautious superintendent at a children's entertainment decided that
it would be safer not to have the exhibition at all, simply because a
regulator was not screwed tightly enough into a cylinder to prevent an
escape of gas, only the operator (a somewhat youthful one) had been driven
to the verge of lunacy by continual questions of the standard type, 'Are
you sure it is safe?' 'Will it blow up?' 'Are you certain you understand
it?' &c., &c. More serious accidents, such as the entire lantern getting
upset, ought never to occur, and it is up to the lanternist to take
whatever precautions he deems necessary to safeguard his instrument. With a
juvenile audience, for example, it is often a good thing to arrange a
barricade of forms round the lantern and to see that no one comes within
it.

Finally, 'whatever is worth doing at all is worth doing well,' and this is
as true of lantern exhibiting as of anything {87} else. There are a
deplorable number of lantern exhibitions given with the sheet hanging in
creases, dirty lenses, light poorly adjusted and centred, and occasionally
slides shown upside down. A conscientious lanternist should see to _every_
detail; slipshod methods, as in everything else, mean poor results.

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