mote
from the common centre of gravity, they may have condensed into masses
of molten matter before the general distribution of them has greatly
altered. In short, as the conditions in each case determine, the
discrete masses produced may vary indefinitely in number, in size, in
density, in motion, in distribution.

And now let us return to the visible characters of nebulae, as observed
through modern telescopes. Take first the description of those nebulae
which, by the hypothesis, must be in an early stage of evolution.

Among the "_irregular nebulae_," says Sir John Herschel, "may be
comprehended all which, to _a want of complete and in most
instances even of partial resolvability_ by the power of the
20-feet reflector, unite such a deviation from the circular or
elliptic form, or such a want of symmetry (with that form) as
preclude their being placed in class 1, or that of Regular Nebulae.
This second class comprises many of the most remarkable and
interesting objects in the heavens, _as well as the most extensive
in respect of the area they occupy_."

And, referring to this same order of objects, M. Arago says:--"The forms
of very large diffuse nebulae do not appear to admit of definition; they
have no regular outline."

This coexistence of largeness, irregularity, and indefiniteness of
outline, with irresolvability, is extremely significant. The fact that
the largest nebulae are either irresolvable or very difficult to resolve,
might have been inferred _a priori_; seeing that irresolvability,
implying that the aggregation of precipitated matter has gone on to but
a small extent, will be found in nebulae of wide diffusion. Again, the
irregularity of these large, irresolvable nebulae, might also have been
expected; seeing that their outlines, compared by Arago with "the
fantastic figures which characterize clouds carried away and tossed
about by violent and often contrary winds," are similarly characteristic
of a mass not yet gathered together by the mutual attraction of its
parts. And once more, the fact that these large, irregular, irresolvable
nebulae have indefinite outlines--outlines that fade off insensibly into
surrounding darkness--is one of like meaning.

Speaking generally (and of course differences of distance negative
anything beyond average statements), the spiral nebulae are smaller than
the irregular nebulae, and more resolvable; at the same time that t