The observed values in general fall between those for the pure
classes and those corresponding to random orientation. They are of
the same order as the latter, and the discrepancies are perhaps not
unaccountably large in view of the nature and the limited extent of the
material. There is a systematic difference, however, averaging about
0.2 mag., in the sense that the observed values are too large, and
increasing with decreasing ellipticity. One explanation is that the
observed classes are purer than is expected on the assumption of random
orientation. This view is supported by the relatively small dispersion
in C, as may be seen in Table I and Figure 2, among the nebulae of a
given class, but it is difficult to account for any such selective
effect in the observations. The discrepancies may be largely eliminated
by an arbitrary adjustment of the numbers of nebulae with various
degrees of actual ellipticity; for instance, the values in the last
column of Table XII, calculated on the assumption of equal numbers,
agree very well with the observed values, although the resulting
numbers having the various apparent ellipticities differ slightly from
those observed. The observed values, however, can again be accounted
for by the inclusion of some flatter nebulae among the classes E6 and
E7. Very early Sa or SBa nebulae might easily be mistaken for E nebulae
when oriented edge-on, although they would be readily recognized when
even slightly tilted. If the numerical results fully represented actual
statistical laws, the explanation would be sought in the physical
nature of the nebulae. The change from ellipsoidal to lenticular
figures, noticeable in the later-type nebulae, would affect the results
in the proper direction, as would also a progressive shortening of the
polar axis. The discrepancies, however, are second-order effects, and
since they may be due to accidental variations from random orientation,
a further discussion must await the accumulation of more data.
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