The effect of the orientation is appreciable among the spirals in
general. In order to illustrate this feature, they have been divided
into three groups consisting of those whose images are round or nearly
round, elliptical, and edge-on, or nearly so. The mean values of m_{T}
+ 5 log d were then computed and compared with the theoretical value,
13.0. The residuals are negative when the nebulae are too bright for
their diameters and positive when they are too faint. The results are
given in Table IX, where mean residuals are followed by the numbers of
nebulae, in parentheses, which are represented by the means.
The numbers of the barred spirals are too limited to inspire confidence
in the results, but among the normal spirals there is conclusive
evidence that the highly tilted and edge-on nebulae are fainter for a
given diameter than those seen in the round. A study of the individual
images indicates that the effect is due very largely to dark absorption
clouds, which become more conspicuous when the nebulae are highly
tilted. These clouds are generally, but not universally, peripheral
features. An extensive investigation will be necessary before any
residual effect due to absorption by luminous nebulosity can be
established with certainty. Even should such exist, it clearly cannot
be excessive.
[Illustration: Fig. 6.—Progressive characteristics in the sequence of
types. The upper curve represents the progression in total magnitude
with type for nebulae having maximum diameters of one minute of
arc. The elliptical nebulae and the normal spirals are included as
representing the normal sequence, but the barred spirals and the
irregular nebulae are omitted. The figures give the number of objects
observed in each type. Among the later elliptical nebulae the numbers
are so small that means of adjacent types have been plotted. The lower
curve represents the progression in diameter along the normal sequence
for nebulae of the tenth magnitude.]
TABLE VIII
=======+=========+==========
Type | C | Δ log d
-------+---------+----------
E0 | 10.30 | +0.54
1 | 10.65 | .47
2 | 11.00 | .40
3 | 11.35 | .33
4 | 11.70 | .26
5 | 12.05 | .19
6 | 12.40 | .12
7 | 12.75 | +0.05
| |
Sa | 13.31 | –0.06
Sb | 13.90 | .18
Sc | 14.45 | .29
| |
SBa | 13.00 | .00
SBb | 13.16 | .03
SBc | 14.41 | .28
| |
Irr | 13.68 | –0.14
------+---------+----------
[Illustration: Fig. 7.—Relation between luminosity and diameter among
extra-galactic nebulae. The nebulae have been reduced to a standard
type, S0, which, being the mean of E7, Sa, and SBa, represents a
hypothetical transition point between elliptical nebulae and spirals.
The Magellanic Clouds have been included in order to strengthen the
brighter end of the plot.