Transcriber’s Notes:

  Underscores “_” before and after a word or phrase indicate _italics_
    in the original text.
  Equal signs “=” before and after a word or phrase indicate =bold=
    in the original text.
  Illustrations and tables have been moved so they do not break up
    paragraphs.
  Small capitals have been converted to SOLID capitals.
  Typographical and punctuation errors have been silently corrected.




                 PHOTOGRAPHIC INVESTIGATIONS OF FAINT NEBULAE


                        THE UNIVERSITY OF CHICAGO PRESS
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                           The University of Chicago
                    PUBLICATIONS OF THE YERKES OBSERVATORY

                               VOLUME IV PART II
                        PHOTOGRAPHIC INVESTIGATIONS OF
                                 FAINT NEBULAE

                                      BY
                                EDWIN P. HUBBLE

                                [Illustration]

                        THE UNIVERSITY OF CHICAGO PRESS
                               CHICAGO, ILLINOIS

                               COPYRIGHT 1920 BY
                           THE UNIVERSITY OF CHICAGO
                              All Rights Reserved
                            Published January 1920

                            Composed and Printed By
                        The University of Chicago Press
                           Chicago, Illinois. U.S.A.




PHOTOGRAPHIC INVESTIGATIONS OF FAINT NEBULAE[1]

BY EDWIN P. HUBBLE


The study of nebulae is essentially a photographic problem for cameras
of wide angle and reflectors of large focal ratio. The photographic
plate presents a definite and permanent record beside which visual
observations lose most of their significance. Perhaps the one field
left for the older method is the measurement of sharp nuclei deeply
enshrouded in nebulosity. New nebulae are now but rarely seen in the
sky, although an hour’s exposure made at random with a large reflector
has more than an even chance of adding several small faint objects to
the rapidly growing list of those already known. About 17,000 have
already been catalogued, and the estimates of those within reach of
existing instruments, based on the ratio of those previously known to
those new in various fields, lie around 150,000.

[1] A dissertation submitted to the Faculty of the Ogden Graduate
School of Science of the University of Chicago in candidacy for the
degree of Doctor of Philosophy.

Extremely little is known of the nature of nebulae, and no significant
classification has yet been suggested; not even a precise definition
has been formulated. The essential features are that they are situated
outside our solar system, that they present sensible surfaces, and
that they should be unresolved into separate stars. Even then an
exception must be granted for possible gaseous nebulae which appear
stellar in the telescope, but whose true nature is revealed by the
spectroscope. It may well be that they differ in kind and do not
form a unidirectional sequence of evolution. Some at least of the
great diffuse nebulosities, connected as they are with even naked-eye
stars, lie within our stellar system; while others, the great spirals,
with their enormous radial velocities and insensible proper motions,
apparently lie outside our system. The planetaries, gaseous but well
defined, are probably within our sidereal system, but at vast distances
from the earth.

In addition to these classes are the numberless small, faint nebulae,
vague markings on the photographic plate, whose very forms are
indistinct. They may give gaseous spectra, or continuous; they may
be planetaries or spirals, or they may belong to a different class
entirely. They may even be clusters and not nebulae at all. These
questions await their answers for instruments more powerful than those
we now possess.

Our present hope is to study them statistically, but until motions,
either radial or transverse, have been detected we must content
ourselves with the problem of their distribution. The first step is
to make a systematic survey with powerful telescopes. Fath made a
beginning by photographing each of the Kapteyn fields within reach of
the Mount Wilson 60-inch reflector with uniform exposures of one hour.
He discovered more than eight hundred new nebulae, and confirmed the
fact that the small nebulae avoid the Milky Way. This last is vital in
its bearing on the question of whether or not these objects belong to
our system. A survey with long exposures suggests itself, analogous to
that of Kapteyn, but based on the Milky Way rather than on the equator.
The writer attempted such a program with the Yerkes 24-inch reflector,
giving two-hour exposures. Little progress was made, but one fact stood
out, namely, that in the fields of galactic latitude -60° nebulae were
very scarce when compared to the numbers met with in galactic latitude
+60°.

The tendency of small nebulae to gather in clusters has been known for
some time. Stratonoff’s map of the distribution of faint nebulae in the
Northern Hemisphere shows it very plainly. Max Wolf’s more detailed
study of the ecliptic regions with the 16-inch Bruce camera and the
30-inch reflector demonstrates that within these larger regions of the
sky where nebulae tend to congregate there are points of accumulation
about which the clustering is more marked. He measured the positions
of more than four thousand new nebulae, and devised a classification
which, while admittedly formal, offers an excellent scheme for
temporary filing until a significant system shall be constructed.

The present paper has to do with certain clusters of small, faint
nebulae which the writer found during the years 1914 to 1916 while
photographing with the 24-inch reflector of the Yerkes Observatory.
From about 1000 uncatalogued objects, 512 in 7 well-defined clusters
were chosen for measurement. Known nebulae in the clusters numbered 76;
hence there were, in all, some 588 objects, or an average of 84 per
cluster. The fields are as given in Table I.

The problem of measuring and reducing accurate positions of objects
at a considerable distance from the center of plates taken with
a reflector of so large a focal ratio, 1:4, presented serious
difficulties. The area covered by each plate is a square of some 110´
to the side. With the full aperture the stellar images are sensibly
round only within 5′ of the optical center of the plate. From there
outward the coma becomes more and more prominent, distorting the images
first into an oval, and finally, near the edge of the plate, into the
shape of an arrow, while the point about which the images build up
becomes more and more eccentric. For images of various sizes this point
will be at various distances from the centers of figure, and at 40′
from the center will fall very nearly at the point of the arrow. This
introduces at once an overwhelming magnitude-error, masking whatever
distortion of the field may exist.


TABLE I

    =======+==============================+=====================
           |        CENTER (1875.0)       |       NUMBER
     FIELD +--------------------+---------+-------+-----+-------
           |         α          |    δ    | Known | New | Total
    -------+--------------------+---------+-------+-----+-------
    I      |  1ʰ    0ᵐ   30ˢ    | +31°44′ |  21   |  57 |   78
    II     |  1    42    20     |  32  0  |   3   |  81 |   84
    III    | 11     3    54     |  29 27  |   8   | 178 |  186
    IV     | 13    37    10     |  56 21  |  21   |  52 |   73
    V      | 14    57    10     |  23 47  |   3   |  49 |   52
    VI     | 17    11    22     |  43 50  |   5   |  43 |   48
    VII    | 23    14    16     |   7 27  |  15   |  52 |   67
           +--------------------+---------+-------+-----+-------
      Total|                    |         |  76   | 512 |  588
    -------+--------------------+---------+-------+-----+-------

If very faint stellar images could be used for reference, this error
could be largely reduced. It was necessary, however, to use stars from
the catalogues of the Astronomische Gesellschaft for reference, and
with the long exposures required for the faint nebulae the images of
these stars were very large. At the edge of the plate, for instance,
the arrow-shaped image of a star of the ninth magnitude would often be
fully a minute of arc in length.

It seemed inadvisable to make an exhaustive study of this
magnitude-error, whence the alternatives were to use a restricted
portion of the field or to sacrifice accuracy in the reduced positions.
The second of these evils was chosen. The positions of the optical
centers of images at various distances from the center of the field
were determined empirically. Pairs of plates of a region were taken
with apertures of 9 inches and with the full 24 inches and were
compared in the Zeiss “blink” comparator. With the smaller aperture,
and hence the smaller focal ratio, the images near the edge of the
plates were sensibly round and small. Superimposed on the 24-inch
images, they indicated where the wires should be set in measuring the
larger distorted images. Trials were then made, measuring positions of
A.G. stars all over the 24-inch plates, until a kind of technique was
acquired. Judged by the aims in view and the results obtained, this
empirical scheme fully justified itself.

At least two plates of each field were taken. In any case the two best
plates were put on the “blink” comparator, and only those objects
clearly nebulous on both plates were marked. The better plate was then
placed on a Gaertner measuring machine. The nebulae and all the A.G.
stars fainter than the seventh magnitude were measured in _X_ and _Y_
with the same screw. After an interval of a day or two the plate was
remeasured. Settings were read to 0.01 mm, corresponding to about 0.87"
on the scale of the plate. The two measures of a nebula differed but
seldom in this unit, and if faint reference stars could have been used,
a higher degree of accuracy could have been maintained throughout the
work.

In order to orient the plate, two stars were selected with as large a
difference in right ascension and as small a difference in declination
as possible. The difference in _X_ was then computed in millimeters,
assuming a scale-value of 87.4″ per mm. The plate was placed in the
measuring machine with the meridian roughly perpendicular to the screw,
and adjusted with the tangent slow-motion until the setting on the two
stars gave the computed difference to the limit of accuracy of the
settings. This method reduced the constant of orientation of the plate
to an almost negligible quantity.

Turner’s method (_The Observatory_, 16, 373, 1893) was used as the
basis for reducing the measures. The six plate-constants for each field
were determined from the five or six A.G. stars most symmetrically
distributed about the optical center, and graphs were constructed
from which the values of _Y_-_Y₀_ and _X_-_X₀_ were corrected. The
Δα and Δδ were then added directly to the α₀ and δ₀ of the assumed
center, and final corrections to the positions were read from graphs
constructed from Turner’s formulae. Any A.G. stars not included in the
determination of plate-constants furnished checks.

Since the distribution of the stars was about the same as that of the
nebulae, it was hoped that the reduced positions of the latter would
be of the same order of accuracy as those of the stars. For the 62
A.G. stars on the seven plates measured, the average difference from
the A.G. positions was 1.0″ in either co-ordinate. The settings on the
nebulae could be made with greater precision than on the stars, hence
these results justify placing the accuracy of the nebular positions at
about 2.0″ in either co-ordinate, except for such as were near the edge
of the plates.

The positions are given for the epoch of the A.G. catalogues, 1875.0.
Two of the N.G.C. nebulae had been measured by Lorenz from photographs
made at Heidelberg. A comparison of his measures with the present
measures shows the same second of arc in declination and the same tenth
of a second of time in right ascension. The agreement is perfect to the
last units used in the present paper, but as the nebulae, N.G.C. 7619
and 7626, are situated near the center of the plate, they cannot serve
as a test of the accuracy of those near the edge. In several cases
comparisons could be made with positions from visual measures, as given
in the _Strassburg Annals_, Vols. III and IV. Here the agreement is not
so good.

In one case, on the same night, positions of four objects were measured
with reference to a certain star, which was, in turn, tied up to an
A.G. star some distance away. The objects are given as N.G.C. 3550,
3552, 3554, and a nova which is designated as K₁₂. The photograph shows
N.G.C. 3550 and 3554 properly placed with reference to the star, both
in distance and in position-angle; there is nothing at all in the place
given for N.G.C. 3552; K₁₂ is properly placed from the star, but is in
the N.G.C. position for No. 3552. The position of the reference star
is given as about 17″ too small in declination. K₁₂ is clearly N.G.C.
3552, and evidently the fourth object does not exist in the published
position. As these objects were all in the same field of view in the
telescope, one is at a loss to account for the discrepancy. A list of
the comparisons is given in Table II.

The descriptions indicate form, brightness, and size, and occasionally
the location of a neighboring star. Wolf’s classification was used.
It is, as he remarks, wholly empirical and probably without physical
significance, yet it offers the best available system of filing away
data, and will later be of great service when a significant order
is established. One class was interpolated between g and h, and was
designated g₀. Brightness was estimated in the order B, pB, pF, F, vF,
eF, eeF, and eeeF. The range is from Herschel’s Class II down to the
limit of the plates. For several of the fields diameters were estimated
to the nearest 5″. Otherwise the size was given in the order L, pL, pS,
cS, S, vS, eS.

The classification given in Table III is illustrated in Plate III,
copied from Wolf’s engraving in Band III, No. 5, of the _Publicationen
des Astrophysikalischen Instituts Königstuhl-Heidelberg_. The most
striking feature is the great predominance of the classes e and f.
These two classes form a continuous sequence from the brightest in the
list to the very limits of the plates, where they are but mere faint
markings on the films. Eleven are clearly spirals, and the spindles are
unexpectedly common. These results are typical.

The frequency of the classes e and f may merely be a way of stating
that the scale of the telescope is too small to show the ordinary
structure, but it must be remembered that many members of these classes
are pretty large and bright and that the gradation in the series is
apparently continuous. As far as telescopes of moderate focal length
are concerned, the predominant form of nebulae as we know them at
present is not the spiral, but is this same “e, f” class, described
as round or nearly so, brightening more or less gradually toward
the center, and devoid of detail. The brightest of the class are
probably Messier 60 and N.G.C. 3379. Their spectrum, as derived from
objective-prism plates, is continuous, and is probably of the same type
as those of the spirals and the globular clusters. A detailed study
on an adequate scale of the brighter members of the class will throw
considerable light on the problem of the small nebulae.


TABLE II

    -----------------------------------------
          HUBBLE _minus_ STRASSBURG
    --------+----------------+---------------
    N.G.C.  |      Δα        |       Δδ
    --------+----------------+---------------
    379     |     0ˢ.0       |      + 3″
    380     |     +0.1       |      + 3
    382     |     +0.2       |      + 1
    383     |     +0.1       |        0
    384     |     +0.1       |      + 1
    385     |     +0.1       |      + 2
    386     |     -0.2       |      - 2
    387     |     +0.2       |      + 3
    3550[2] |     +0.5       |      +24
    3552    |     +0.1       |      +19
    3554    |     -0.2       |      +15
    3558    |     -0.3       |      - 1
    6329    |     -0.3       |      + 3
    6332    |     -0.3       |      + 6
    6336    |     +0.1       |      + 8
    7586    |     +0.2       |      + 1
    7608    |     -0.3       |      + 6
    7611    |     -0.2       |      - 1
    7612[3] |      0.0       |      + 2
    7617    |     +0.1       |      + 1
    7619[4] |     +0.1       |      - 1
    7623[5] |      0.0       |      + 1
    7626[6] |     -0.2       |      - 2
    --------+----------------+---------------

[2] Nucleus is eccentric and undefined on the photograph, hence the
photographic position is probably in error by several seconds of arc.

[3] Mean of the positions given in Vols. III and IV. N.G.C. 7621 is
5ˢ.4 preceding, and 1′ 49″ south of 7623. There is a double star in the
position published in the _Strassburg Annals_.

[4] Mean of the positions given in Vols. III and IV. N.G.C. 7621 is
5ˢ.4 preceding, and 1′ 49″ south of 7623. There is a double star in the
position published in the _Strassburg Annals_.

[5] Mean of the positions given in Vols. III and IV. N.G.C. 7621 is
5ˢ.4 preceding, and 1′ 49″ south of 7623. There is a double star in the
position published in the _Strassburg Annals_.

[6] Mean of the positions given in Vols. III and IV. N.G.C. 7621 is
5ˢ.4 preceding, and 1′ 49″ south of 7623. There is a double star in the
position published in the _Strassburg Annals_.

TABLE III

DISTRIBUTION OF VARIETIES OR CLASSES OF NEBULAE IN THE SEVEN CLUSTERS
OR FIELDS

    ======+===========================================
          |                  FIELD
    CLASS +----+----+-----+----+----+----+-----+------
          |  I | II | III | IV |  V | VI | VII | Total
    ------+----+----+-----+----+----+----+-----+------
    a     |    |    |   1 |    |    |    |     |   1
    c     |    |  1 |     |    |  1 |    |  1  |   3
    d     |    |  1 |  33 |  1 |    |    |  6  |  41
    e     | 27 | 39 | 109 | 41 | 27 | 21 | 20  | 284
    f     | 33 | 35 |  17 | 19 |  8 | 12 | 21  | 145
    g     |  7 |  3 |   4 |    | 12 |  5 |  7  |  38
    g₀    |  5 |    |   8 |    |    |  3 |  1  |  17
    h     |  2 |  2 |   2 | 10 |  1 |  1 |  4  |  22
    h₀    |  3 |  2 |   4 |    |  1 |  2 |  2  |  14
    i     |    |    |     |    |    |  1 |  2  |   3
    k     |  1 |    |   2 |    |    |    |     |   3
    n     |    |    |     |    |    |    |  1  |   1
    q     |    |  1 |     |    |    |    |  2  |   3
    r     |    |    |     |    |    |  2 |     |   2
    v     |    |    |     |    |    |  1 |     |   1
    w     |    |  1 |   4 |    |  2 |    |     |   7
    irreg |    |    |   1 |  2 |    |    |     |   3
    ------+----+----+-----+----+----+----+-----+------

Messier 60 has a spiral as a very near neighbor--H III 44. The contrast
in the two classes is well shown; so also in the case of N.G.C. 3379
mentioned above. Here there is a group of three fairly bright nebulae:
N.G.C. 3379, a globular nebula of class e; 3389, an open spiral; and
3384, which might be called a spindle, except that the wings flare out
from the nucleus.

The three irregular forms are N.G.C. nebulae and are commented upon in
the descriptions accompanying the positions.

Very little can be said concerning the surface-brightness of
these objects. It is independent of the distance so long as the
angular diameter is sensibly greater than that of a faint star. The
photographic plate therefore records the absolute surface-brightness
of the nebulae. High luminosity is a comparatively rare attribute and
there is some relation between luminosity and absolute size; that is
to say, the brighter usually have the greater angular diameter. Since
it is hard to conceive of a relation between distance and absolute
brightness, the fact that the faint nebulae are usually the smaller can
be interpreted only in the light of a relation between luminosity and
absolute size.

The clustering of the nebulae here recorded is very pronounced. In the
center of Field III there are some 75 nebulae scattered over an area
equal to that subtended by the full moon.[7] In order to examine the
distribution of the different sizes, diameters were plotted against
frequencies. The small scale of the plates renders the number of
smallest diameters very uncertain. Nebulae less than 10″ in diameter
might easily be mistaken for stars and overlooked, especially if they
are at some distance from the center of the field. The curves resemble
probability-curves, as one would expect for random distribution in a
cluster at a definite distance.

[7] See Plate IV, enlarged from negative R 3352, taken with 120ᵐ
exposure on February 26, 1916. The numbers were marked on only those
nebulae which promised to be readily visible on the engraving, and
which were separated enough to give room for inscribing the number. The
B.D. stars are designated by letters, for which the key is as follows.

FIELD III

    STAR   B.D.

     A = +30°2107
     B = +30°2108
     C = +30°2109
     D = +30°2110
     E = +30°2115
     F = +30°2121
     G = +30°2123
     H = +29°2123
     J = +29°2125
     K = +29°2126
     L = +29°2128
     M = +29°2129
     N = +29°2130
     P = +29°2133
     R = +29°1970
     S = +28°1971


TABLE IV

DISTRIBUTION ACCORDING TO SIZE OF THE NEBULAE IN THREE FIELDS

    =========+===================+==============
             |      FIELD        |
    DIAMETER +-------------------+ WOLF’S FIELD
             |  I  |  III |  VII |  IN PERSEUS
    ---------+-----+------+------+--------------
     5″      |     |      |      |      3
    10       |  7  |  38  |   5  |     34
    15       | 27  |  67  |  12  |     52
    20       | 25  |  42  |  18  |     26
    25       |  6  |  19  |  10  |      6
    30       |  5  |   9  |   4  |      2
    35       |  3  |   2  |   4  |      1
    40       |  4  |   5  |   4  |      2
    45       |  1  |   3  |   3  |
    50       |  1  |      |   3  |
    60       |     |      |   3  |
    90       |     |      |   1  |
    ---------+-----+------+------+--------------

Exposures of two hours gave all of the nebulae recorded. Five-hour
exposures brightened the images somewhat, but revealed no new ones. The
conclusion would seem to be that the limits of the clusters had been
reached. This, however, is uncertain, for the longer exposures gave
relatively few stars which were not on the plates of shorter exposures.
As a matter of fact, the 24-inch reflector at this altitude seems to
have a maximum working efficiency at slightly over two hours. Save
for very exceptionally clear and steady skies, the longer exposures
add much labor for negligibly greater results. The diameters of the
faintest stars near the center of the field, with the full aperture,
fine sky, and an hour’s exposure, which are of about magnitude 17.5,
are about 2.0″. Longer exposures are apparently subject to change of
focus, differential refraction, and other disturbances which tend to
increase the size of the images unduly, and hence to spread the total
light over a larger area. The result is that the value of _p_ in the
reciprocity equation _Itᵖ_ = _iTᵖ_ does not remain constant throughout
the exposure, but varies, beyond a certain value of _T_, depending on
the adjustments of the telescope, the position of the field, and the
condition of the sky. However, the effect should be more noticeable on
the stars than on nebulae which present surfaces.

[Illustration: FIG. 1.--Distribution of size in Wolf’s Nebel Listen
Nos. 3 to 14]

I have plotted Wolf’s lists of nebulae, Nos. 3-14, in the same manner,
converting estimates of size into seconds of arc according to his
table. These lists were made from plates taken with the 16-inch (41 cm)
Bruce camera, of focal length 203 cm, of the Heidelberg Observatory.
The curves (Fig. 1) take the same form, save that for most the maximum
frequency is for diameters between 20″ and 25″. One of his lists was
made from plates taken with the 30-inch reflector at Königstuhl. It
is of a field in Perseus, α = 3ᵸ12ᵐ, δ = +41° 6′. Diameters of the
124 Wolf nebulae and five others were measured from plates taken with
the Yerkes 24-inch reflector. This plot (Fig. 2) gives a maximum for
diameters around 15″, and the longer focus of Wolf’s 30-inch apparently
does not add to the number of small nebulae distinguishable on the
plates made with the shorter telescope.


TABLE V

WOLF’S NEBEL LISTEN NOS. 3-14

    =====+=================================================
         |                  Diameter
    List +-----+-----+------+------+------+-------+--------
         |  4″ |  6″ |  15″ |  25″ |  60″ |  200″ | >200″
    -----+-----+-----+------+------+------+-------+--------
     3   | 205 | 322 |  291 |  280 |  195 |  36   |   6
     4   |     |   1 |   69 |  153 |   19 |   6   |   1
     5   |     |   6 |   99 |  106 |   23 |   1   |   3
     6   |     |  14 |  114 |   72 |    2 |   2   |   1
     7   |     |   9 |  103 |  156 |   35 |   4   |  12
     8   |     |  80 |  372 |  243 |   34 |   3   |
     9   |     |  48 |  174 |  160 |   19 |       |
    10   |     |   3 |   31 |   26 |    2 |       |
    11   |     |     |   13 |   60 |   19 |   1   |
    12   |     |  14 |   61 |  162 |   27 |  10   |
    13   |     |     |   20 |   61 |   26 |   3   |
    14   |     |   3 |  160 |  296 |   43 |   1   |   1
    -----+-----+-----+------+------+------+-------+--------

The evidence, while far from conclusive, appears to indicate the
existence of actual clusters of these small nebulae in the sky. If this
is true, it is natural to suppose them physically connected, as is the
case in star-clusters. It is not possible to form a conception of this
state of affairs until some idea of their distance is acquired. Suppose
them to be extra-sidereal and perhaps we see clusters of galaxies;
suppose them within our system, their nature becomes a mystery.

[Illustration: FIG. 2.--Distribution of size of nebulae in Wolf’s
Perseus and three Yerkes fields]

The question of nebular distances is of first importance, for it is in
terms of this quantity that the various dimensions may be expressed.
The dark nebulosities, by their very nature, and the great diffuse
clouds, some obviously connected with even naked-eye stars, may safely
be considered as galactic, and this view is in accord with their low
radial velocities with reference to our system.

The planetaries have repeatedly been measured for proper motion, with
negligible results. Taking a value of 40 km/sec. for the average radial
velocity, and an assumed lower limit of 0.02″ for the average annual
proper motion, a tentative lower limit for the average distance of the
largest, and hence probably the nearest, is found to be about 2000
light-years. There is thus no reason on this ground for placing them
outside our system, especially in view of their decidedly systematic
galactic distribution.

Rotation of these nebulae, as detected by the spectroscope, furnishes
a means of relating mass and average density with distance. Assume
an axis perpendicular to the line of sight, the mass as concentrated
in the nucleus and the individual distant particles as rotating in
equilibrium; let α be the radius, _P_ the period of rotation, _M_ the
mass, and suppose the rotation to be circular. Then α³/_P²M_ = _C_, a
constant. Let the unit of distance be the light-year (LY); of time,
the year; of mass, that of the sun (S), then _C_, as computed from the
earth-sun system, is about 4 × 10⁻¹⁵.

Let

     α  = the angular radius in seconds of arc
    _d_ = the distance in light-years
     ρ  = the density in terms of earth’s atmosphere at sea-level
    _v_ = the linear velocity of rotation in km/sec.

Then the following relations hold:

    (1) _M_ = 3.4 × 10⁻⁴ _d_α_v_²
    (2)  ρ  = 1.4 × 10⁻⁶ (_v_/_d_α)²
    (3) _P_ = 9.15 _d_α/_v_

The velocity of escape for the nebula is proportional to α/ρ and
hence to _v_. This follows from the assumption that the particles are
rotating in equilibrium, and therefore the factor of proportionality is
the ratio between parabolic and circular velocity, that is, 1.4, and is
independent of the distance. The value of _v_ for those nebulae so far
observed is small, ranging from 5 to 10 km. Hence, if the assumptions
held only approximately, the velocity of escape would be small and
of the same order as that for the earth. Since these nebulae are
composed of the lightest gases, it follows that at any save very low
temperatures the molecules would escape at a very rapid rate. Certainly
the nebulae would dissipate if the temperatures were of the order of
that of our own atmosphere.


TABLE VI

    =======+==========+=========+=================+==================
     _d_   | Diameter |   Mass  |     Period      |    Density
    -------+----------+---------+-----------------+------------------
    10 LY  | 0.001 LY |    1.2S | 1.5×10² year    |  1.8×10⁻⁷ρ
    10²    | 0.01     |   12.   | 1.5×10³         |  1.8×10⁻⁹
    10³    | 0.1      |  120.   | 1.5×10⁴         |  1.8×10⁻¹¹
    10⁴    | 1.0      | 1200.   | 1.5×10⁵         |  1.8×10⁻¹³
    -------+----------+---------+-----------------+------------------

For an assumed typical planetary nebula, 20″ in diameter, rotating
with a velocity of 6 km at 10″ from the perpendicular axis, Table VI
has been constructed from formulae (1)-(3), expressing the order of
magnitude of dimensions in terms of distance.

The velocity of escape would be about 8.4 km per second, whatever the
distance.

Spectroscopic rotation of spirals furnishes an analogous set of
formulae, and here the inclination of the axis may be roughly
determined from the ratio of the two diameters of the nebulae. Let β be
the semi-minor axis, then the formulae will be:

    (4) _M_ = 3.4 × 10⁻⁴ _d_α_v_²

                   α        (  _v_ )²
    (5) ρ = 1.4 × --- × 10¹²(------) in suns per cu. LY, or
                   β        ( _d_α )

                   α        (  _v_ )²
          = 2.8 × --- × 10⁻⁶(------) in atmospheres
                   β        ( _d_α )

                        α
    (6) _P_ = 9.15 _d_ ---
                       _v_

The spirals form a continuous series from the great nebula of Andromeda
to the limit of resolution, the smaller ones being much the more
numerous. Considering them to be scattered at random as regards
distance and size, some conception may be formed of their dimensions
from the data at hand. The average radial velocity of those so far
observed is about 400 km, while the proper motion is negligible.
Putting the annual proper motion at 0.05″, the lower limit of the
average distance is found to be about 7500 light-years. If they are
within our sidereal system, then, as they are most numerous in the
direction of its minor axis, the dimensions of our system must be much
greater than is commonly supposed.

The observations point to very large values for the rotational
components of velocity, although the necessarily small scale of the
instruments employed in their study renders the measuring difficult.
Pease has determined the velocity of rotation for N.G.C. 4594 with
some degree of accuracy. At 120″ from the nucleus it amounts to 300 km
and varies linearly with the distance outward. V. M. Slipher reports
that for the Andromeda nebula the angular rotation is fastest near the
nucleus, and that this type of rotation promises to be the more common.

Assume a typical spiral 400″ in diameter, with the ratio of the axes
of figure as β/α = 0.1, and with rotation perpendicular to the line of
sight at a velocity of, say, 200 km at the periphery. These figures
are apparently not very different from the average of the two dozen
brighter spirals. Table VII gives the dimensions in terms of distance.

The velocity of escape is 280 km/sec.

At the lower limit of average distance of spirals the typical nebulae
would be fifteen light-years in diameter, forty-five million times
as massive as our sun, and 3 × 10¹³ as dense as our atmosphere. On
the other hand, if the typical spiral nebula is placed at a suitable
distance, its dimensions assume the same order of magnitude as those of
our own stellar system.

The conception of our galaxy set forth by Eddington in his book
_Stellar Movements and the Structure of the Universe_ is that the Milky
Way forms a ring around a central, slightly flattened cluster. This
ring is supposed to rotate in equilibrium so that the stars may remain
concentrated in the configurations they now form. Assuming the ratio
of the two radii as one to five, and using Eddington’s figures of 2000
parsecs for the distance of the Milky Way, and 10⁹ suns as the mass of
the inner cluster, the period and density may be computed and compared
with those of the typical nebula placed at a distance such as will make
the diameter the same as that of our system. The results are given in
Table VIII.

TABLE VII

    ======+========+============+==========+==========================
       d  |    α   |    Mass    |  Period  |         Density
    ------+--------+------------+----------+--------------------------
      10² |   10⁻¹ |  2.7 × 10⁵ |  9 × 10² | 1.4 × 10⁹ suns per cu. LY
      10³ |   10⁰  |  2.7 × 10⁶ |  9 × 10³ | 1.4 × 10⁷
      10⁴ |   10¹  |  2.7 × 10⁷ |  9 × 10⁴ |  1.4 × 10⁵
      10⁵ |   10²  |  2.7 × 10⁸ |  9 × 10⁵ |  1.4 × 10³
      10⁶ |   10³  |  2.7 × 10⁹ |  9 × 10⁶ |  1.4 × 10¹
    ------+--------+------------+----------+--------------------------

TABLE VIII

    =======+==========+=========+============+==============+===========
           | Distance | Radius  |    Mass    |   Period     |  Density
    -------+----------+---------+------------+--------------+-----------
    Nebula |6 × 10⁶ LY|6 × 10³LY|1.6 × 10¹⁰ S|5.4 × 10⁷ year| 5.6 × 10⁻¹
    Galaxy |          |6 × 10³  |      10⁹   | 2.5 × 10⁸    | 1.2 × 10⁻²
    -------+----------+---------+------------+---------------+----------

The velocities of escape would be about 280 km for the nebula and 170
km for the galaxy. Considering the problematic nature of the data,
the agreement is such as to lend some color to the hypothesis that
the spirals are stellar systems at distances to be measured often in
millions of light-years. The computations by O. H. Truman[8] and by R.
K. Young and W. E. Harper[9] of the motion of our system with respect
to the spirals, based on the radial velocities of the spirals, are
another and stronger argument for the hypothesis.

[8] _Popular Astronomy_, =24=, 111, 1916.

[9] _Journal of the R.A.S., Canada_, =10=, 134, 1916.

The principal objection lies in their apparently systematic
distribution with respect to the Milky Way. The matter is usually
stated in the form that “spirals avoid the Milky Way.” There are less
than 300 nebulae _known_ to be spiral in form. The greatest of them all
is just on the border. It is suggested that the spirals seem to follow
the distribution of the small faint nebulae. If this is true, the most
that can be definitely stated is that they tend to cluster in certain
regions of the sky, in one of which the north pole of the galaxy is
located.

As the small nebulae and the spirals inhabit the same regions of the
sky, it is probable that the order of distance of the two classes is
the same. Classes e, f, may even turn out to be spirals themselves.
This, however, is a question for large instruments, and is outside the
scope of the present paper.

TABLE IX

FIELD I OF NEBULAE

    ---+-------------------------------+-----+--------------------------
       |            (1875.0)           |     |
    No.|------------------+------------|CLASS|      DESCRIPTION
       |         α        |      δ     |     |
    ---+------------------+------------+-----+--------------------------
     1 | 0ᴴ  56ᵐ   35.1ˢ  | +31°36′39″ |  f  | pF, R, 20″d, *14m 20″p.
     2 |          39.3    |  31 38 24  |  e  | F, R, 20″d, *12m 20″n.
     3 |          49.0    |  31 22 14  |  h₀ | F, mE110°, 60″l.
     4 | 0   57    5.7    |  32  4 12  |  e  | eF, st. *13m 30″np.
     5 |          54.5    |  32 17 41  |  e  | vF, R, 25″d, *14m 30″sf.
     6 | 0   58    8.9    |  32  8 33  |  e  | vF, R, 25″d, *9 1′nf.
     7 |          22.3    |  31 33 12  |  e  | eF st.
     8 |          41.8    |  31 45 33  |  f  | pF, R, S, Δ2 faint*, bM.
     9 |          42.7    |  31 17 58  |  f  | pF, st. 2*13, 14 1.5′p.
    10 |          54.7    |  31 16 30  |  g  | F, cE0°, 30″l.
    11 |          58.0    |  31 43  5  |  g  | F, cE130°, 30″l, bM.
    12 |          59.7    |  32  5 33  |  e  | vF, R, 30″d.
    13 | 0   59   13.9    |  31 56 21  |  f  | vF, st. *16m40″sf.
    14 |          28.2    |  31 31 43  |  f  | vF, st. *15m 1.5′f.
    15 |          51.6    |  32 33 48  |  g  | vF, E140°, 45″l.
    16 |          58.2    |  32  1 58  |  f  | vF, E60°, 20″l 3f*.
    17 |          58.8    |  31 41 27  |  f  | eF, eS.
    18 | 1    0    0.7    |  31 54 35  |  e  | F, st.
    19 |           3.6    |  31 18 39  |  e  | vF, st. d. nuc.
    20 |           6.4    |  31 29 19  |  e  | vF, 1E, 20″l.
    21 |           8.2    |  31 42 16  |  e  | vF, st.
    22 |          10.9    |  31 43 12  |  h  | vF, E90°, 1′l.
    23 |          15.0    |  32 24  1  |  e  | eF, eS 2*10, 11m1′nf.
    24 |          18.9    |  31 30 22  |  g  | vF, cE, S, *15m40″s.
    25 |          22.0    |  31  1 42  |  e  | vF, st.
    26 |          22.1    |  32 22 51  |  e  | eF, R, 30″d, *12m1.5′nf.
    27 |          22.9    |  31 31 29  |  f  | FcE60°1′l, *16m40″np.
    28 |          25.0    |  31 45 17  |  f  | eF, E, eS.
    29 |          29.0    |  31 54 48  |  h₀ | Fv, mE160°, 40″l*13m1′s.
    30 |          33.3    |  31 54 41  |  e  | eF, E, eS.
    31 |          39.3    |  31 25 23  |  h  | F, mE130°, 1′l.
    32 |          43.2    |  32 17 18  |  e  | vF, R, 40″d, *14m30″s.
    33 |          59.1    |  31 35 14  |  g  | eF, eS, st.
    34 | 1    1    3.5    |  32 38  9  |  e  | vF, cE40°*11m1′nf.
    35 |          11.3    |  31 13 16  |  h₀ | vF, 310°, 1′l.
    36 |          19.6    |  31 47  9  |  f  | F, S, E60°, *14m1′s.
    37 |          24.5    |  31 25 17  |  f  | F, st. *14m1′np.
    38 |          27.8    |  31 52 56  |  f  | eF, vS, *15m20″p.
    39 |          32.7    |  31 45 48  |  e  | eF, st. in line with 2f*.
    40 |          50.3    |  31 45 57  |  e  | eF, iR, *14m1′s.
    41 |          58.8    |  31 25 54  |  e  | pF, R, 25″d. no nuc.
    42 | 1    2    5.5    |  31 34 21  |  e  | vF, st. *10m20″s.
    43 |          20.4    |  31 29  4  |  e  | vF, R, 45″d. no nuc.
    44 |          22.3    |  31 18 42  |  f  | F, st. and sev f*.
    45 |          26.4    |  32  3 22  |  e  | vF, st. *11m20″s.
    46 |          32.0    |  32  6 19  |  e  | eF, iR*14m30″sf.
    47 |          39.7    |  31 30 21  |  g  | vF, cE80°, S.
    48 |          48.5    |  31 47  5  |  f  | F, st. *15m1′nf.
    49 |          49.0    |  31 41 39  |  f  | eF, vS, *15m30″f.
    50 |          49.8    |  31 57  3  |  f  | eF, st. *14m1′np.
    51 | 1    3    5.3    |  31 42  7  |  f  | vF, st. 2*14m1′f.
    52 |           8.0    |  32 14 51  |  e  | eF, st.
    53 |          26.4    |  31 43 18  |  f  | vF, st. Δ2vF*.
    54 |          37.6    |  31 30 51  |  f  | eF, st. bet. 2*.
    55 |          57.6    |  31 55 41  |  f  | eF, st. *12m15″p.
    56 | 1    4   16.4    |  32  2  9  |  f  | vF, st. bet. 2*.
    57 | 1    5    1.6    |  31 48 22  |  e  | eF, st. *12m1′s.
    ---+------------------+------------+-----+--------------------------

NEBULAE PREVIOUSLY KNOWN IN FIELD I

    ------+---------------+------------+----+---------------------------
    N.G.C.|               |            |    |
    370   | 0ᴴ  59ᵐ 51.6ˢ | +31°44′55″ | g  | vF, S, cE20°* 14m30″s.
    374   | 1    0  12.5  |  32  7 35  | g₀ | pB, mE10°bet. 2*13m.
    376   |         14.3  |  31 40 43  | f  | vF.
    379   |         22.5  |  31 51  8  | g₀ | pB, cE 0°, 60″×30″.
    380   |         24.5  |  31 48 53  | e  | pB, R, 40″d.
    382   |         30.9  |  31 44  8  | f  | pB, R, 20″d.
    383   |         32.0  |  31 44 38  | e  | pB, R, 1'd bM.
    384   |         32.2  |  31 37 26  | g₀ | pB, cE135°, 30″l.
    385   |         34.3  |  31 39  4  | f  | pB, R, 40″d.
    386   |         38.3  |  31 41 35  | f  | pF, st.
    388   |         54.2  |  31 38 28  | f  | F, st.
    392   | 1    1  29.1  |  32 27 59  | f  | pF, R, 30″d bM, *11m1′ sp.
    394   |         31.6  |  32 28 50  | f  | F, st.
    397   |         36.4  |  32 26 32  | f  | vF, st.
    398   | 1    2   0.0  |  31 50 50  | f  | F, st.
    399   |          5.2  |  31 58  1  | g₀ | F E50° 40″l.
    403   |         20.0  |  32  5  9  | k  | pB, mE90°, 60″×20″.
          |               |            |    |
    387   | 1   0   40.1  |  31 43 23  | f  | vF, eS, st.
          |               |            |    |
    I.C.  |               |            |    |
    1618  | 0  59    3.4  |  31 44 31  | g₀ | pF, cE, 150° 25″l, bM.
    1619  | 1   0   28.6  |  32 23 57  | f  | pF, st. bet. 2*11, 12m.
    ------+---------------+------------+----+---------------------------

    N.G.C. 379 and 372 are probably the same object, with α of 370,
    and δ the mean of the two N.G.C. positions. There is no other object
    in the immediate vicinity.

    400 }
    401 } Faint stars in these positions; no nebulae near.
    402 }
    390, Faint star, 16m. in this position. No trace of a nebula.

TABLE X

FIELD II OF NEBULAE

    ===+===============================+=====+===========
       |          (1875.0)             |     |
    No.+-------------------+-----------+CLASS|DESCRIPTION
       |       α           |     δ     |     |
    ---+-------------------+-----------+-----+-----------
     1 |    1ᴴ 39ᵐ 32.1ˢ   |+31°52′58″ |  f  |F, S.
     2 |           41.7    | 32 46  3  |  e  |eF, pS.
     3 |           43.4    | 32  4 25  |  d  |eeF, S.
     4 |           46.7    | 31 29  2  |  f  |vF, S.
     5 |           50.4    | 32  7 35  |  e  |eF, pS.
     6 |           55.6    | 31 24 29  |  e  |eF, eS.
     7 |           59.0    | 32 10 21  |  f  |vF, vS.
     8 |    1   40 27.5    | 31 24 17  |  h₀ |eeF, S.
     9 |           48.3    | 31 30 29  |  e  |eeF, vS.
    10 |           50.4    | 31 37 46  |  e  |eeF, vS.
    11 |    1   41  5.6    | 31 44  9  |  e  |eF, S.
    12 |            8.9    | 31 55 48  |  e  |vF, vS.
    13 |            9.4    | 31 59 34  |  f  |vF, pS.
    14 |           20.2    | 31 53 23  |  e  |eF, vS.
    15 |           32.6    | 32 20  2  |  f  |vF, S.
    16 |           37.1    | 31 55 40  |  e  |eF, vS.
    17 |           47.6    | 32 16 38  |  f  |eF, vS.
    18 |           51.5    | 31 55 50  |  e  |eeF, S.
    19 |    1   42  0.5    | 31 56 38  |  e  |eeeF S.
    20 |            0.8    | 32 28 25  |  f  |vF, S.
    21 |           15.8    | 32 11 28  |  g  |eF, 30″l.
    22 |           16.4    | 31 52 43  |  e  |eeF, eS.
    23 |           21.1    | 31 55 41  |  e  |eeF, eS.
    24 |           23.5    | 31 57 24  |  f  |F, S.
    25 |           31.5    | 32  0  9  |  g  |vF, S.
    26 |           37.8    | 32 42 19  |  e  |eeF, S.
    27 |           41.4    | 32 34 24  |  e  |eeF, S.
    28 |           43.5    | 31 29 20  |  f  |eeF, eS.
    29 |           44.4    | 32 10 20  |  g  |pF, 30″l.
    30 |           50.4    | 32 19 55  |  f  |vF, S.
    31 |           50.9    | 32 47 16  |  e  |eF, S.
    32 |           51.2    | 31 24 50  |  e  |eeF, eS.
    33 |           58.0    | 32  0  4  |  e  |eeF, eS.
    34 |    1   43  2.7    | 32 52 33  |  f  |eF, S.
    35 |            5.3    | 31 51 52  |  e  |eeeF, eS.
    36 |            7.0    | 31 48 17  |  f  |pF, S.
    37 |            7.5    | 32 32  4  |  w  |eF,40″d.
    38 |            7.7    | 31 56 36  |  f  |vF, vS.
    39 |            8.6    | 31 54 15  |  e  |eeF, eS.
    40 |            9.1    | 31 53  1  |  f  |eF, eS.
    41 |            9.3    | 32  6 11  |  e  |eeF, vS.
    42 |           12.0    | 31 52 35  |  f  |eeF, eS.
    43 |           12.2    | 32 22 58  |  f  |eF, S.
    44 |           13.1    | 31 51 39  |  h  |eeF, S.
    45 |           13.2    | 31 59 34  |  e  |eeF, vS.
    46 |           13.4    | 32  2 32  |  e  |eeF, eS.
    47 |           17.9    | 31 57 14  |  f  |vF, vS.
    48 |           19.6    | 32 25 39  |  f  |vF, vS.
    49 |           20.7    | 31 50  3  |  h  |eeeF, S.
    50 |           22.6    | 32 36  9  |  e  |eeF, vS.
    51 |           22.6    | 31 49 17  |  e  |eeF, S.
    52 |           23.8    | 31 47 57  |  e  |eeF, eS.
    53 |           29.5    | 31 52 29  |  f  |vF, S.
    54 |           30.5    | 32 27 43  |  f  |pF, pS.
    55 |           37.6    | 31 53 20  |  e  |eF, eS.
    56 |           39.0    | 31 56 43  |  f  |eF, eS.
    57 |           39.2    | 32 28 21  |  f  |vF, S.
    58 |           39.3    | 32 13 46  |  e  |eeF, vS.
    59 |           39.9    | 31 53  5  |  e  |eeF, eS.
    60 |           43.4    | 31 53 22  |  e  |eeF, eS.
    61 |           46.8    | 31 52 18  |  f  |eeF, eS.
    62 |           48.9    | 32 16 44  |  f  |eF, vS.
    63 |           56.4    | 32 18 51  |  f  |eF, vS.
    64 |           58.8    | 32  0 10  |  e  |eeeF, vS.
    65 |    1   44  0.7    | 32 27 29  |  e  |eeF, vS.
    66 |            0.8    | 32  9 58  |  e  |eeF, eS.
    67 |            1.9    | 32  0 17  |  f  |vF, vS.
    68 |            4.0    | 32 29  4  |  e  |eeeF, S.
    69 |            8.4    | 32 24  6  |  e  |eF, vS.
    70 |           11.3    | 32 24 47  |  f  |F, pS.
    71 |           14.9    | 32 12 48  |  e  |eeF, vS.
    72 |           20.7    | 32 25 54  |  f  |vF, S.
    73 |           24.6    | 32  4 37  |  f  |eeF, eS.
    74 |           24.6    | 32  9 38  |  f  |eF, eS.
    75 |           34.9    | 32  5 34  |  f  |eeF, eS.
    76 |           43.5    | 32 27 34  |  e  |eeF, pS.
    77 |           47.5    | 31 33 13  |  f  |eF, eS.
    78 |           52.7    | 32 24 24  |  e  |eF, vS.
    79 |           56.0    | 32 14 24  |  e  |eeeF, vS.
    80 |           57.4    | 32  4  2  |  e  |eeF, S.
    81 |    1   46 28.6    | 31 22  2  |  f  |eeF, eS.
    ---+-------------------+-----------+-----+-----------

NEBULAE PREVIOUSLY KNOWN IN FIELD II

    =====+================+=============+===+======================
    I.C. |                |             |   |
    1733 | 1ᴴ  43ᵐ 25.6ˢ  |+31° 56′ 40″ | f |vF, eS.
    1735 |         35.5   | 31  55  32  | c |vF nuc., iR eeF neb.,
         |                |             |   |  60″d.
         |                |             |   |
         | 1   42  20.4   | 31  57  55  | q | 270″×30″. Found
         |                |             |   |  visually by Barnard.
         |                |             |   |  Not catalogued.
    -----+----------------+-------------+---+----------------------

TABLE XI

FIELD III OF NEBULAE

    ----+---------------------------+-----+--------------------------
        |            (1875.0)       |     |
    No. |----------------+----------|CLASS|    DESCRIPTION
        |        α       |     δ    |     |
    ----+----------------+----------+-----+--------------------------
     1  | 10ᴴ  59ᵐ 59.4ˢ |+29°22′40″| e   | eeF, 25″d.
     2  | 11    0   2.1  | 29 23 37 | e   | 35″d.
     3  |          14.7  | 29 46 56 | e   | eF, 30″d.
     4  |          22.0  | 29 15 59 | e   | vF, 20″d.
     5  |          23.5  | 29  5 59 | f   | eF, 30″d.
     6  |          25.8  | 29 27 21 | f   | vF, 30″d.
     7  |          42.5  | 28 39 11 | e   | ef, 15″d.
     8  |          56.1  | 29 20 10 | d   | eeF, 30″d.
     9  | 11    1   1.0  | 28 51  3 | e   | eF, 20″d.
    10  |           5.2  | 29 10 31 | e   | eF, 25″d.
    11  |          19.5  | 29 22 46 | e   | eeF, 15″d.
    12  |          22.4  | 28 39 12 | w   | eF, 30″d, open spiral.
    13  |          23.9  | 29 12 21 | e   | vF, 25″d.
    14  |          24.8  | 29 19  4 | h₀  | eeF, E165°, 35″×10″
    15  |          27.3  | 29 32 18 | d   | eeF, 15″d.
    16  |          28.4  | 29  5 32 | e   | vF, 30″d.
    17  |          28.4  | 29 12 58 | d   | eF, 20″d.
    18  |          29.2  | 29 31 11 | e   | eF, 15″d.
    19  |          32.3  | 29  6 30 | h₀  | eeF, 3160°.
    20  |          36.6  | 29 17 52 | d   | eeF, 20″ d, some structure.
    21  |          38.6  | 29 17 13 | f   | eF, 20″d.
    22  |          53.4  | 29 19 33 | e   | eF, E60°, 30″×15″.
    23  |          56.6  | 29 54  3 | d   | eF, 30″d.
    24  | 11   2    2.2  | 29 36 46 | d   | eeF, 15″d.
    25  |           5.7  | 29 50  3 | w   | eeF, 45″d, open spiral.
    26  |          17.2  | 28 57 27 | e   | eF, 20″d.
    27  |          24.0  | 28 45  9 | g   | eF, E160°, 40″×20″.
    28  |          30.3  | 29 33  6 | e   | eeF, 10″d.
    29  |          36.0  | 31 14 51 | e   | vF, 25″d.
    30  |          36.4  | 29 43 59 | e   | eeF, 10″d.
    31  |          37.9  | 29 24  6 | d   | eeF, 20″d.
    32  |          38.7  | 29 26 44 | e   | eeF, 15″d.
    33  |          38.8  | 29 44 12 | e   | eeF, 15″d.
    34  |          39.8  | 29 20 17 | e   | eeF, 15″d.
    35  |          40.3  | 29 54 55 | e   | eeF, 15″d.
    36  |          41.2  | 29 27 20 | w   | eeF, nuc. 10″d, ring, 30″d.
    37  |          41.4  | 28 55 57 | e   | eF, 15″d.
    38  |          42.8  | 29 23 45 | d   | vF, 15″d.
    39  |          44.9  | 29 23 51 | e   | eF, 10″d.
    40  |          47.3  | 29 18 17 | k   | vF, E150°, 45″×15″.
    41  |          48.0  | 29 43 23 | g₀  | eeF, E40°, 20″×10″.
    42  |          48.3  | 29 21 21 | e   | eF, 15″d.
    43  |          54.0  | 29 21 19 | g₀  | eF, 30″×10″.
    44  |          55.7  | 29 35 33 | e   | F, 20″d.
    45  |          56.6  | 28 55 41 | e   | eF, 35″d.
    46  |          58.2  | 29 13 50 | g   | eeF, E50°, 30″×10″.
    47  | 11   3    4.1  | 29 10 39 | a   | eeF, 15″d, structure.
    48  |           4.3  | 29 38 56 | g₀  | eeF, E80°, 20″×10″.
    49  |          21.6  | 29  1 14 | d   | eF, 15″d.
    50  |          22.6  | 29 18 15 | d   | eeF, 20″d.
    51  |          23.4  | 29 17 21 | e   | vF, 20″d.
    52  |          23.6  | 29 39 35 | e   | eF, 20″d.
    53  |          24.3  | 29 40 11 | e   | eeF, 20″d.
    54  |          27.9  | 29  9  3 | f   | eF, 15″d.
    55  |          28.6  | 30  7  9 | f   | eeF, 30″d.
    56  |          28.7  | 29  0 47 | d   | eeF, 15″d.
    57  |          30.1  | 28 49  9 | e   | eeF, 20″d.
    58  |          31.4  | 29 43 22 | e   | F, 15″d.
    59  |          31.5  | 29 28  6 | e   | eF, 15″d.
    60  |          32.5  | 29 14 55 | e   | eF, 15″d.
    61  |          33.4  | 28 57 49 | e   | eF, 15″d.
    62  |          35.8  | 29 18  9 | f   | vF, 20″d.
    63  |          38.5  | 29 23 55 | e   | eeF, 10″d.
    64  |          39.6  | 29 23  9 | a?  | eF, 35″×25″. E35°,
        |                |          |     |   a miniature Dumb-bell.
    65  |          39.7  | 29 25 39 | e   | eeeF, 10″d.
    66  |          40.1  | 29 25 48 | e   | eeeF, 10″d.
    67  |          40.3  | 29 23 57 | e   | eeF, 10″d.
    68  |          41.3  | 29 21 55 | g₀  | eF, E110°, 30″*10″.
    69  |          41.9  | 29 22 21 | e   | eF, 20″d.
    70  |          43.7  | 30  7 14 | e   | eeF, 20″d.
    71  |          44.7  | 29 22 38 | e   | eeF, 20″d.
    72  |          46.2  | 29 28  7 | f   | eF, 20″d.
    73  |          46.4  | 29 30 25 | d   | eF, 15″d.
    74  |          49.8  | 29 22 46 | d   | eeF, 20″d.
    75  |          51.6  | 29 22 53 | e   | eeF, 10″d.
    76  |          52.4  | 29 22 34 | e   | eeF, 15″d.
    77  |          52.9  | 29 43 59 | e   | eeF, 20″d.
    78  |          53.6  | 28 59 39 | e   | F, 35″d.
    79  |          54.5  | 29 26  1 | d   | eeF, 20″d.
    80  |          54.7  | 29 23  5 | e   | eeF, 20″d.
    81  |          55.1  | 29 21 50 | e   | eF, 15″d.
    82  |          56.7  | 29 26 22 | e   | eeF, 15″d, E50°.
    83  |          56.9  | 29 34 40 | e   | eF, 15″d.
    84  |          57.5  | 29 16 59 | e   | eF, double nebula,
        |                |          |     |         nuc. 4″ apart.
    85  |          57.6  | 29  8  7 | e   | eF, 30″d.
    86  |          57.7  | 29 27 19 | h₀  | eeF, E140°, 15″×5″.
    87  | 11   4    0.8  | 29 17 51 | e   | eeF, 10″d.
    88  |           1.0  | 28 56 28 | e   | eeF, 20″d.
    89  |           1.4  | 29 22 15 | e   | eF, 15″d.
    90  |           1.4  | 29 39  5 | e   | eeF, 15″d.
    91  |           1.8  | 28 57 21 | e   | vF, 30″d.
    92  |           3.4  | 29 27 26 | e   | eF, 10″d.
    93  |           3.7  | 29 29 10 | e   | eeF, 20″d.
    94  |           2.7  | 29 20 17 | e   | vF, 20″d.
    95  |           3.8  | 29  2  9 | e   | eeF, iR.
    96  |           4.4  | 29 27 33 | e   | eF, 10″d.
    97  |           5.7  | 29 28 23 | e   | eeF, 15″d, faint extensions.
    98  |           5.8  | 30 14 51 | e   | eF, st.
    99  |           6.9  | 29 22 50 | e   | eF, 15″d.
    100 |           7.6  | 28 56 51 | h₀  | eeF, E25°, 30″×10″.
    101 |           9.4  | 29 14 23 | e   | eeF, 15″d.
    102 |           9.9  | 29 29 29 | d   | eeF, 10″d.
    103 |          10.0  | 28 53 55 | e?  | eeF, faint extensions
        |                |          |     |             60″×40″?
    104 |          10.1  | 30  0 27 | d   | eF, 30″d.
    105 |          10.6  | 29  8 48 | g₀  | eF, E145°, spiral, 40″×20″.
    106 |          12.8  | 29 28 58 | h   | eeeF, 20″×10″.
    107 |          13.4  | 29 23 15 | e   | eeF, 15″d.
    108 |          13.5  | 29 21 19 | g₀  | eeF, E125°, 25″×15″.
    109 |          13.8  | 29 47 14 | d   | eeF, 20″d.
    110 |          14.2  | 30  0 57 | k   | eF, E40°, 34″×20″.
    111 |          14.4  | 29 25 24 | e   | eF, 15″d.
    112 |          15.6  | 29 29 36 | d   | eeeF, 10″d.
    113 |          18.6  | 28 55 56 | e   | eeF, 15″d.
    114 |          20.2  | 28 53  6 | f   | eeF, 20″d.
    115 |          22.8  | 28 54  6 | e   | eeF, 20″d.
    116 |          27.9  | 29 23 25 | f   | eF, 35″d.
    117 |          27.9  | 29 26 40 | e?  | eeF, 60″×40″, spiral?
    118 |          28.4  | 29 22 31 | e   | eF, 45″d.
    119 |          28.6  | 29 21 58 | d   | eeF, 15″d.
    120 |          28.6  | 29 25 18 | e   | eeF, 10″d.
    121 |          30.2  | 29 37 42 | e   | eeF, 15″d.
    122 |          32.0  | 29 25 37 | d   | eeF, 15″d.
    123 |          34.4  | 29 21 33 | e   | eF, 20″d.
    124 |          34.9  | 29 42  1 | e   | eF, 20″d.
    125 |          35.5  | 29 12 10 | e   | eeF, 10″d.
    126 |          36.0  | 29 21  0 | e   | eF, E60°, 20″×15″.
    127 |          37.0  | 29 27  7 | d   | eeeF, 15″d.
    128 |          38.5  | 28 56 22 | e   | eF, st.
    129 |          39.4  | 29 24 33 | f   | vF, 25″d.
    130 |          40.2  | 29 12 57 | e   | eeF, 15″d.
    131 |          41.8  | 29 27 40 | g   | eeF, E95°, 20″ × 10″.
    132 |          46.8  | 29 19 18 | g   | eeF, 30″ × 15″.
    133 |          47.9  | 29 31  9 | d   | eeF, 10″d.
    134 |          49.3  | 29 26  1 | f   | eF, 15″d.
    135 |          49.9  | 29 29 19 | e   | eeF, 10″d.
    136 |          49.9  | 29 14 45 | e   | eeF, 15″d.
    137 |          50.5  | 29 25 27 | e   | eF, 30″d.
    138 |          51.5  | 29 22 55 | e   | eF, 10″d.
    139 |          52.7  | 29 23 21 | e   | eF, 10″d.
    140 |          54.1  | 29 29 57 | e   | eeF, 15″d.
    141 | 11   5    3.0  | 28 56 43 | e   | vF, 30″d.
    142 |           4.7  | 29 16 36 | e   | eF, 15″d.
    143 |           6.8  | 29 15 53 | e   | eeF, 20″d.
    144 |           7.6  | 28 53  4 | d   | eeF, 15″d.
    145 |          11.6  | 29 38  5 | e   | eeF, 20″d.
    146 |          12.3  | 29 10  1 | d   | eF, 20″d.
    147 |          13.5  | 30  9 30 | d   | eF, 20″d.
    148 |          14.4  | 30  6 28 | e   | eF, 20″d.
    149 |          14.7  | 29 20 38 | w   | eF, 25″d, spiral.
    150 |          15.6  | 29  8 19 | d   | eeeF, 15″d.
    151 |          17.1  | 29 29 16 | e   | eF, 20″d.
    152 |          19.4  | 29 10 52 | g   | vF, E40°, 50″ × 20″.
    153 |          34.3  | 29 25 10 | d   | eeF, 30″d.
    154 |          35.5  | 29 15  0 | e   | eeF, 30″d.
    155 |          43.6  | 28 57 24 | e   | eF, 40″d.
    156 |          49.2  | 28 42 34 | f   | eF, st.
    157 |          51.3  | 28 43 12 | e   | eeF, 30″d.
    158 |          59.6  | 28 57 38 | e   | eeF, 30″d.
    159 | 11   6    6.9  | 28 44 55 | d   | eeF, 30″d.
    160 |           9.7  | 29 29 30 | d   | eeeF, 20″d.
    161 |          10.6  | 28 40 46 | e   | eF, 30″d.
    162 |          11.8  | 29  4  8 | f   | vF, st.
    163 |          12.8  | 30 11 16 | e   | eeF, 10″d.
    164 |          17.5  | 29 25 47 | e   | eF, 15″d.
    165 |          23.2  | 28 43 55 | f   | eeF, 20″d.
    166 |          24.9  | 28 39 40 | e   | eeF, 30″d.
    167 |          27.8  | 28 41 26 | e   | eeF, 30″d.
    168 |          29.0  | 28 56 22 | e   | eeF, 20″d.
    169 |          40.9  | 29 20 27 | d   | eeeF, 20″d.
    170 |          37.7  | 28 33 23 | d   | eeeF, 15″d.
    171 |          42.7  | 29 37 59 | e   | eeF, 20″d.
    172 |          44.2  | 29  5 13 | d   | eeeF, 15″d.
    173 |          50.6  | 29 18 31 | e   | eeF, 20″d.
    174 | 11   7   11.1  | 28 32 54 | d   | eeF, 20″d.
    175 |          14.9  | 30 14 45 | e   | eF, 20″d.
    176 |          19.2  | 29 18 25 | h   | eeeF, 30″×10″.
    177 |          27.1  | 29  6 26 | f   | eF, 20″d.
    178 |          32.9  | 28 51 30 | e   | eF, 30″d.
    ====+================+==========+=====+====================

NEBULAE PREVIOUSLY KNOWN IN FIELD III

    =========================================================
    N.G.C.|             |           |   |
    3527  |11ᴴ 0ᵐ 31.4ˢ |+29° 12′ 6″| f | vF, 35″d.
    3536  |11  2    5.3 | 29   9  5 | e | F, 40″d.
    3539  |        22.9 | 29  20 56 | g₀| F, E5°, 60″×20″.
    3550  |11  3   53.5 | 29  26 47 |   | pB, eccentric nuc., 35″ × 25″.
    3552  |11  3   52.2 | 29  24 38 | e | F, 20″d.
    3554  |11  3   57.7 | 29  22 15 | e | vF, 25″d.
    3558  |11  4   10.9 | 29  13 17 | f | vF, 15″d, with what appears
          |             |           |   |  to be a faint ring 50″ in d.
    3561  | 11 4   28.4 | 29  22 31 | e | eF, 45″d.
    ------+-------------+-----------+---+-------------------------

TABLE XII

FIELD IV OF NEBULAE[10]

    ===+==============================+=====+=================
       |           (1875.0)           |     |
    No.|------------------------------|CLASS|   DESCRIPTION
       |        α         |     δ     |     |
    ---+------------------+-----------+-----+-----------------
     1 |13ᴴ   32ᵐ   33.0ˢ |+56°18′28″ |  e  | F, eS, *15m10″n.
     2 |13    33     0.4  | 56 49 46  |  e  | eeF, pS.
     3 |            16.0  | 55 49 23  |  e  | eF, S.
     4 |            41.7  | 57  9  1  |  e  | eF, S.
     5 |            55.6  | 56  5  4  |  e  | vF, eS.
     6 |            56.9  | 56  6 15  |  e  | eF, S.
     7 |13    34     9.1  | 55 50 46  |  e  | eeF, eS.
     8 |            11.3  | 56 32  8  |  f  | eF, eS.
     9 |            25.9  | 56 29 14  |  f  | eF, eS.
    10 |            56.6  | 56 42 36  |  e  | eF, vS.
    11 |            57.7  | 57  0 46  |  f  | eF, vS.
    12 |13    35     4.9  | 56 45 21  |  h  | eeF, S.
    13 |            49.8  | 56 13 38  |  h  | vF, S.
    14 |            52.0  | 56 52 58  |  e  | eF, cS.
    15 |            53.1  | 56  3 25  |  e  | eeF, eS.
    16 |13    36    20.1  | 55 50 25  |  e  | eeF, eS.
    17 |            29.1  | 56 51 25  |  e  | eF, S.
    18 |            33.1  | 55 49 59  |  e  | eeF, eS.
    19 |            33.2  | 56 43 54  |  f  | eF, eS.
    20 |            38.6  | 55 46 18  |  e  | eeF, eS.
    21 |            39.0  | 56 18 33  |  h  | F, 30″l.
    22 |            41.0  | 56 12  2  |  d  | eeF, S.
    23 |            43.7  | 56 42 11  |  e  | eeF, S, *15m10″s.
    24 |            46.2  | 56 48 12  |  e  | eF, cL.
    25 |            49.7  | 56 41 26  |  f  | eF, S.
    26 |            53.6  | 56 48 57  |  e  | eeF, eS.
    27 |            55.7  | 56 27 19  |  h  | eeF, S.
    28 |            56.1  | 56 40 17  |  e  | eeF, S.
    29 |13    37     7.2  | 56  4 29  |  f  | vF, S.
    30 |            11.6  | 55 46 36  |  e  | eeF, eS.
    31 |            19.0  | 56 24 54  |  e  | eF, S.
    32 |            20.2  | 56 26 51  |  f  | eeF, vS.
    33 |            35.5  | 57  7  6  |  e  | eF, cS.
    34 |            46.0  | 56  5  6  |  f  | eeF, S.
    35 |            54.0  | 56 10 12  |  h  | eeF, S.
    36 |            58.2  | 56  9 31  |  h  | eeF, S.
    37 |13    38     4.3  | 56  7 32  |  f  | eeF, S.
    38 |             9.8  | 56  7 11  |  f  | eeF, eS.
    39 |            15.8  | 56 13  9  |  e  | eeF, eS.
    40 |            17.1  | 56 13 57  |  f  | eF, vS.
    41 |            29.1  | 56 16 49  |  e  | eF, S.
    42 |            35.2  | 56 15  5  |  h  | eF, vS.
    43 |            35.4  | 56 13 55  |  h  | eeF, eS.
    44 |            37.3  | 56 14 41  |  f  | eF, vS.
    45 |            47.3  | 56 41 19  |  e  | eeF, cS.
    46 |            52.8  | 55 35 23  |  e  | eeF, eS.
    47 |            56.4  | 56 12 58  |  f  | eF, vS.
    48 |13    39     6.9  | 56 15 45  |  e  | eeF, eS.
    49 |             7.9  | 56 16 31  |  h  | eF, vS.
    50 |             8.1  | 56  5 45  |  f  | vF, S.
    51 |             9.2  | 56 11 45  |  e  | eeF, eS.
    52 |             9.4  | 55 56 14  |  e  | eeF, eS.
    53 |            13.3  | 56 16 32  |  f  | eeF, vS.
    54 |            17.6  | 56 10 33  |  e  | eeF, eS.
    55 |            19.8  | 56 10 50  |  e  | eeF, eS.
    56 |            23.0  | 56 11 40  |  e  | eF, eS.
    57 |            25.1  | 56 16 43  |  e  | eeF, eS.
    58 |            30.3  | 56 26 49  |  f  | eF, eS.
    59 |            31.6  | 56 20  3  |  e  | eeF, S.
    60 |            45.1  | 56 29  5  |  f  | eF, vS.
    61 |            45.7  | 56 34 46  |  e  | eeF, S.
    62 |            54.2  | 56 15 18  |  e  | eeF, eS.
    63 |13    40     3.2  | 56 37 22  |  e  | eeF, eS.
    64 |             5.9  | 56 30 56  |  e  | eeF, eS.
    65 |             5.9  | 56 30 29  |  h  | eeF, 30″l.
    66 |            27.2  | 56  6 27  |  e  | eeF, vS.
    67 |13    41    27.2  | 56 20 38  |  e  | eF, 60″d.
    68 |            38.8  | 55 47 15  |  e  | eF, eS.
    69 |            47.4  | 55 44 54  |  f  | eF, vS.
    70 |13    42    40.2  | 56 15 54  |  e  | eeF, S.
    ---+------------------+-----------+-----+-----------------

[10] Field IV covers the position of a group of 18 small nebulae
announced by E. E. Barnard in _Astronomische Nachrichten_, 125, 369,
1890. The positions there given were rough estimations from the stars
B.D. +56°.1679 and B.D. +56°.1682. On the photographs, the nebulae
in this region are so small and so crowded that I have been able to
identify only three individuals of the group. Barnard’s Nos. 4, 7, and
18 are very probably my Nos. 41, 43, and 62.

NEBULAE PREVIOUSLY KNOWN IN FIELD IV

    ======+==================+=============+===+===============
    N.G.C.|                  |             |   |
    5278  | 13ᴴ  36ᵐ  59.45ˢ |+56°18′ 3.8″ |   |
    5279  |      37     3.72 | 56 18 14.5  |   |
    5294  |      40    39.7  | 55 55  2    | f |eF, S, *15m1′np.
    ------+------------------+-------------+---+----------------
        N.G.C. 5278 and 5279 form a double nebula, somewhat
        similar to Messier 51. 5278 is the nucleus and 5279
        is at the tail of the arm. The spiral apparently
        has but one branch.

TABLE XIII

FIELD V OF NEBULAE

    ====================================================================
        |          (1875.0)         |     |
    No. |---------------------------|CLASS|  DESCRIPTION
        |        α      |     δ     |     |
    ----+---------------+-----------+-----+-----------------------------
     1  | 14ᴴ 53ᵐ 52.5ˢ |+23°10′16″ | e   | eF, S, R.
     2  |         54.5  |  23 57 39 | e   | vF, R, 20″d, *18m40″nf.
     3  | 14  54  56.2  |  23 53 37 | g   | eF, S, m3. *16m1′.np.
     4  |         58.1  |  23 16 56 | e   | vF, pS, iR.
     5  | 14  55   6.7  |  23 24 32 | f   | eF, S.
     6  |         16.2  |  24  5 30 | f   | F, S, *17m30″n.
     7  |         31.4  |  24  6  9 | w   | F, 30″d. Spiral.
     8  |         35.9  |  23 58 27 | g   | vF, mE, 40″l, bet. 2*.
     9  |         37.3  |  24 12 53 | e   | eF, eS.
    10  | 14  56  13.7  |  23 52 21 | g   | F, E, spiral?
    11  |         17.7  |  24  2 30 | g   | F, E180°.
    12  |         26.3  |  24  0 51 | h₀  | vF, S, iR.
    13  |         27.0  |  24  9 28 | g   | F, vS, E.
    14  |         28.6  |  23 23  8 | e   | vF, S, R, 15″d, no nuc.
    15  |         30.0  |  23 54 53 | e   | vF, vS, R.
    16  |         32.6  |  24  2 47 | f   | eF, vS.
    17  |         46.4  |  24  3 54 | e   | eF, S, iR.
    18  |         47.8  |  24 36  4 | e   | vF, S, iR.
    19  |         51.2  |  23 40  4 | e   | vF, vS.
    20  |         56.0  |  23 51 10 | e   | eF, vS, R.
    21  |         58.7  |  23 49 12 | f   | eF, vS, R.
    22  |         58.7  |  23 50 14 | e   | eF, eS, R.
    23  |         59.0  |  23 51 41 | g   | eF, S, IE.
    24  | 14  57   4.3  |  23 51 51 | e   | eF, S.
    25  |          4.3  |  23 52  8 | f   | eF, E.
    26  |          6.1  |  24  5 56 | f   | F, bet. 2*14, 15m.
    27  |          8.4  |  23 47 22 | g   | eF, cS, iR.
    28  |          8.9  |  24 16 59 | e   | eF, S.
    29  |         10.3  |  23 50 31 | e   | eF, S.
    30  |         11.3  |  23 46 44 | e   | eF, S.
    31  |         11.8  |  23 49 57 | e   | eF, S.
    32  |         13.6  |  23 53  2 | h   | eF, S.
    33  |         18.8  |  23 44 22 | g   | eF, S.
    34  |         24.1  |  23 42 42 | e   | eF, cS, iR.
    35  |         24.3  |  24  4 40 | e   | eF, S, iR.
    36  |         26.8  |  23 45  2 | e   | eF, vS, iR.
    37  |         35.9  |  24  6 37 | e   | eF, S.
    38  |         41.7  |  24 35  7 | f   | eF, S.
    39  |         43.0  |  23 24 38 | e   | vF, R, 30″d, no nuc.
    40  |         51.7  |  24  0 48 | g   | eF, vS, Δ with 2*14 and 16m.
    41  | 14  58   0.6  |  23 18  4 | c   | vF, 25″d, spiral?
    42  |          6.8  |  23 11 21 | g   | F, cL, mE180°, 80″×15″.
    43  |         18.5  |  23 26 30 | g   | eF, S, 20″d.
    44  |         19.8  |  23 21 55 | f   | vF, R? 60″d.
    45  |         25.9  |  23 25 35 | e   | eF, S, R, 20″d.
    46  |         48.4  |  23 40 57 | g   | eF, cS, mE.
    47  |         54.7  |  23 54 12 | e   | F, mE 180°.
    48  | 14  59  56.8  |  24 10 32 | e   | vF, S, iR, bM.
    49  | 15   0   7.2  |  23 52 11 | e   | eF, S, iR.
    ----+---------------+-----------+-----+-----------------------------

NEBULAE PREVIOUSLY KNOWN IN FIELD V

    =========================================================
    N.G.C.|              |            |   |
    5829  | 14ᴴ 57ᵐ 9.6ˢ | +23°49′29″ | w | open 2br, spiral.
          |              |            |   |
    I.C.  |              |            |   |
    4526  | 14  57   5.9 |  23 50 31  | e | pB, R, 18″d.
    4532  |     59  21.7 |  23 44 43  | e | pB, S, E.
    ------+--------------+------------+---+------------------
        I.C. 4526 is connected with N.G.C. 5829.
        The two form a double nebula fashioned
        as a miniature of Messier 51.

TABLE XIV

FIELD VI OF NEBULAE

    =====+=========================+=====+==============================
         |        (1875.0)         |     |
     No. +--------------+----------+CLASS|     DESCRIPTION
         |       α      |     δ    |     |
    -----+--------------+----------+-----+------------------------------
       1 |17ᴴ  8ᵐ  16.7ˢ|+44° 5′35″| f   | vF, vS, *13m, 1′f.
       2 |         29.9 | 43 26  6 | f   | vF, vS, st.
       3 |         35.2 | 43 23  8 | g   | eF, E 60°, *13m40″sp.
       4 |         52.6 | 42 57 44 | f   | F, S, st.
       5 |17   9    2.6 | 44 18 28 | e   | vF, vS, *16m, 40″s.
       6 |         14.2 | 44 16 11 | e   | eF, S, *16m, 40″f.
       7 |         17.5 | 43 50 51 | f   | vF, sharp nuc. 30″d.
       8 |         33.2 | 43  2 38 | e   | F, pS, *17m, 1′n.
       9 |         35.1 | 44  2 54 | e   | F, vS, *15m, 30″f.
      10 |         41.1 | 43 38 39 | g₀  | pF, S, mE, 60°*14m, 30″p.
      11 |         53.8 | 43 52  2 | e   | vF, vS, *17m, 40″n.
      12 |17  10    3.4 | 44 12 35 | f   | eF, eS, *16m, 30″n.
      13 |         23.3 | 43 49 32 | e   | vF, vS, bet. 2 vf*.
      14 |         25.5 | 43 43  7 | f   | vF, vS.
      15 |         29.1 | 43 44 48 | e   | vF, vS.
      16 |         33.0 | 44  4 43 | g   | vF, S.
      17 |         37.7 | 44  5 32 | e   | eF, vS.
      18 |         39.0 | 43 42 38 | g   | vF, eS, *16m, 40″sf.
      19 |         40.2 | 44  5 20 | h   | eF, vS.
      20 |         46.7 | 43 51 14 | f   | vF, vS, *16m, 40″sf.
      21 |         52.8 | 43 33  9 | f   | vF, S, *16m, 40″sf.
      22 |         55.7 | 44  4  8 | g   | F, cE 70°, *14m, 40″n.
      23 |17  11   16.3 | 43 10 57 | f   | vF, S.
      24 |         25.9 | 43 25 48 | f   | pF, S, *16m, 40″f.
      25 |         33.9 | 43 47 21 | g   | vF, vs, cE 160°, *15m, 40″np.
      26 |         36.7 | 43 42 51 | h₀  | vF, S, cE 120°, faint nuc.
      27 |         43.0 | 43 53 35 | g₀  | pF, S, cE 20°, *14m, 1.5′np.
      28 |         45.7 | 43 55 42 | f   | F, vS, *15m, 40″nf.
      29 |         48.6 | 43 58 50 | e   | vF, S *13m, 1.5′n.
      30 |         57.1 | 44  8 50 | e   | eF, es.
      31 |         57.8 | 44  8  6 | e   | eF, S.
      32 |         57.9 | 43 44 57 | e   | vF, vS, *15m, 30″f.
      33 |17  12   13.3 | 44  2 16 | e   | F, eS, *12m, 1′n.
      34 |         26.2 | 44 12 21 | e   | F, eS.
      35 |         37.2 | 44 12 12 | e   | F, eS, Δ with 2*12m.
      36 |         38.4 | 43 28 52 | f   | pF, vS, *1.5′s.
      37 |         40.9 | 43 54 40 | h₀  | eF, cE 150°, no nuc.
      38 |         44.9 | 43 45 22 | e   | vF, vS, Δ with 2*16m.
      39 |         50.6 | 43 39 48 | f   | vF, eS, Δ with 2 f*.
      40 |         51.0 | 43 48 16 | e   | vF, vS, *16m, 40″nf.
      41 |17  13    4.9 | 43 36 37 | e   | eF, vS, *15m, 20″s.
      42 |         27.2 | 43 40 39 | e   | vF, vS.
      43 |         39.2 | 43 44 19 | e   | vF, S.
    -----+--------------+----------+-----+------------------------------

NEBULAE PREVIOUSLY KNOWN IN FIELD VI

    ======+=================+==========+=====+=========================
    N.G.C.|                 |          |     |
     6323 | 17ᴴ   9ᵐ  30.6ˢ |+43°55′42″|  i  | pF, mE, ns, 40″l.
     6329 | 17   10   27.3  | 43 49 40 |  f  | pF, pL, slE.
     6332 | 17   11   15.2  | 43 48  5 |  g₀ | pF, pL, E 45°.
     6336 | 17   12   30.0  | 43 55 35 |  v  | pF, open spiral, 45″d.
          |                 |          |     |
    I.C.  |                 |          |     |
     4645 | 17   10   53.0  |+43°14′40″|  e  | vF, pS, Δ with 2 faint*.
    ------+-----------------+----------+-----+-------------------------
            N.G.C. 6327 is on the plate but was not measured.

TABLE XV

FIELD VII OF NEBULAE

    =====+========================+=====+===============================
         |        (1875.0)        |     |
     No. +--------------+---------+CLASS|      DESCRIPTION
         |     α        |    δ    |     |
    -----+--------------+---------+-----+-------------------------------
       1 |23ᴴ 10ᵐ  25.7ˢ|+8°13′28″| f   | st. 14m.
       2 |23  11   19.4 | 7 34 13 | e   | F, S, *17m30″s.
       3 |         39.3 | 7 45 26 | d   | vF, R, no nuc., 16m45″ np.
       4 |         47.3 | 7  3 55 | f   | eF, S, R, no nuc.
       5 |23  12   23.1 | 7 18 20 | c   | vF, st. nuc. with ring 45″d.
       6 |         42.2 | 6 45  7 | d   | eF, S, R, no nuc.
       7 |         51.1 | 7  2 10 | q   | F, 1bM, mE100°, 100, ×20″.
       8 |23  13   17.2 | 7 24 12 | h₀  | vF, S, 1E50°*14m30″p.
       9 |         19.4 | 7 34 51 | q   | F, sharp nuc., mE70°, 80″×20″.
      10 |         29.9 | 7 31 15 | f   | F, S, E150°.
      11 |         33.6 | 8  6 51 | d   | eF, pS, R.
      12 |         35.4 | 7 52 39 | g   | pF, bM, mE150° 40″l.
      13 |         41.6 | 7 32  2 | g   | eF, vS.
      14 |         41.6 | 7 18 13 | e   | vF, vS, Δ2 faint *.
      15 |         49.1 | 7  2 16 | e   | vF, S, Δ2 faint *.
      16 |         52.7 | 7 14 52 | h   | F, sharp nuc. vmE20°90″×15″.
      17 |         56.6 | 7 19 16 | e   | eF, pL, no nuc.
      18 |23  14    2.4 | 6 51  8 | f   | F, S, R, *14m90″n.
      19 |          2.9 | 7 38 55 | f   | F, S, R, bM.
      20 |         18.4 | 7 42 40 | f   | F, S, bM.
      21 |         24.4 | 7 45 35 | e   | eF, vS, *16m30″s.
      22 |         31.3 | 7 27 10 | e   | eF, vS, bet. 2*.
      23 |         34.9 | 7 31 47 | e   | vF, S.
      24 |         36.0 | 7 30  3 | f   | vF, vS.
      25 |         36.2 | 7 42  4 | f   | vF, vS.
      26 |         37.5 | 7 41  4 | f   | vF, vS.
      27 |         41.8 | 7 29 40 | h   | FN, mE160°, 80″×20″.
      28 |         41.8 | 7 13 46 | f   | pF, vS, bM, Δ with 2 faint *.
      29 |         42.4 | 7 44 32 | e   | eF, eS,*14m1′sp.
      30 |         46.1 | 7 32 31 | n   | pFN, eccentric, mE90°.
      31 |         46.2 | 7 29 21 | i   | pF, vS, R.
      32 |         52.8 | 6 40 49 | f   | st. 14m.
      33 |         54.9 | 7 32 39 | e   | F, S, 1E.
      34 |         56.2 | 7 10  7 | i   | vF, S, E, *14m30″sf.
      35 |         56.9 | 6 47 41 | e   | vF, 1E, *15m30″s.
      36 |         58.5 | 7 33 24 | e   | vF, vS, *17m30″f.
      37 |23  15    5.9 | 7 51 23 | e   | vF, R, lvM,40″d, *12.1′n.
      38 |         11.1 | 7 20 15 | f   | vF, S, E, *14m30″sp.
      39 |         16.9 | 7 34 23 | e   | vF, vS, *15m1′np.
      40 |         20.4 | 7 38 15 | d   | vF, vS.
      41 |         21.8 | 8 18 24 | h   | vF, E, *9m, superimposed.
      42 |         26.0 | 8 23 37 | d   | pF, pL, R, lbM.
      43 |         28.7 | 7 29 43 | f   | F, S.
      44 |         53.0 | 8 15 14 | e   | eF, pL, iR, no nuc.
      45 |         56.3 | 7 46 32 | e   | eF, S, *16m40″p.
      46 |23  16   24.2 | 7 40 32 | e   | vF, E, *17m40″f.
      47 |         38.8 | 8 18 39 | h₀  | eF, no nuc., vmE175°,
         |              |         |     |         150″×30″.
      48 |         42.5 | 7 33 39 | g   | vF, S, E.
      49 |         43.4 | 7 36 39 | e   | eF, pS, no nuc. Trapz. of 4*.
      50 |         55.3 | 7 51  8 | e   | vF, S, *12m1′s.
      51 |23  17   14.2 | 7 37 37 | e   | eF, vS, *12m2′nf.
      52 |         34.9 | 6 55 40 | d   | eF, pS, no nuc.
    -----+--------------+---------+-----+-------------------------------

NEBULAE PREVIOUSLY KNOWN IN FIELD VII

    ------+---------------+---------+-----+--------------------------
    N.G.C.|               |         |     |
     7604 | 23ᴴ 11ᵐ 31.7ˢ |+6°44′48″| f   | F, R, bM.
     7605 |          32.6 | 6 41 46 | f   | F, R, bM, *15m70″p.
     7586 |          36.1 | 7 54  7 | f   | pF, st.
     7608 | 23  12   55.5 | 7 40  6 | h   | pF, sharp nuc., mE20°,
          |               |         |     |         100″×25″.
     7611 | 23  13   16.6 | 7 22 45 | g₀  | pB, gbM, mE140°, 80″×30″.
     7612 |          24.7 | 7 53 38 | g   | pB, mbM, cE170°, 80″×40″.
     7615 |          35.0 | 7 42 58 | f   | F, E130°, * 14m involved.
     7617 |          49.2 | 7 28 54 | e   | pF, pS, mbM, vlE20°.
     7619 |          54.8 | 7 31 19 | f   | B, R, 90″d.
     7621 | 23   14   5.0 | 7 40 56 | g   | pF, pS, mbM, E0°.
     7623 |          10.4 | 7 42 45 | f   | pB, R, mbM, 60″d.
     7626 |          22.8 | 7 31 56 | f   | B, R, bM, 90″d, *14m60″p.
     7631 | 23  15    7.1 | 7 31 59 | g   | pB, mbM, mE80°, 110′×40″.
     7634 |          22.3 | 8 12 14 | f   | F, R, *10m20″p.
   2d I.C.|               |         |     |
     5309 | 23  12   51.8 | 7 25 32 | g   | pF, mbM, E0°, 50″×30″,
          |               |         |     |   *14m on south edge.
    ------+---------------+---------+-----+--------------------------
      YERKES OBSERVATORY
             May, 1917

[Illustration: PLATE III

WOLF’S CLASSES OF NEBULAE

(Copied from the Königstuhl [Heidelberg] Publications)]

[Illustration: PLATE IV

ENLARGED NEGATIVE OF FIELD III23ᴴ 11ᵐ 31ˢ

Center at α=11ᴴ 4ᵐ, δ=+29°30′

For identification of lettered stars see footnote 7 page 5.]