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.