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are not so small as the regular nebulae, and not so resolvable. This is
as, according to the hypothesis, it should be. The degree of
condensation causing spiral movement, is a degree of condensation also
implying masses of flocculi that are larger, and therefore more visible,
than those existing in an earlier stage. Moreover, the forms of these
spiral nebulae are quite in harmony with the explanation given. The
curves of luminous matter which they exhibit, are _not_ such as would be
described by discrete masses starting from a state of rest, and moving
through a resisting medium to a common centre of gravity; but they _are_
such as would be described by masses having their movements modified by
the rotation of the medium.

In the centre of a spiral nebula is seen a mass both more luminous and
more resolvable than the rest. Assume that, in process of time, all the
spiral streaks of luminous matter which converge to this centre are
drawn into it, as they must be; assume further, that the flocculi, or
other discrete portions constituting these luminous streaks, aggregate
into larger masses at the same time that they approach the central
group, and that the masses forming this central group also aggregate
into larger masses; and there will finally result a cluster of such
larger masses, which will be resolvable with comparative ease. And, as
the coalescence and concentration go on, the constituent masses will
gradually become fewer, larger, brighter, and more densely collected
around the common centre of gravity. See now how completely this
inference agrees with observation. "The circular form is that which most
commonly characterises resolvable nebulae," writes Arago. Resolvable
nebulae, says Sir John Herschel, "are almost universally round or oval."
Moreover, the centre of each group habitually displays a closer
clustering of the constituent masses than the outer parts; and it is
shown that, under the law of gravitation, which we now know extends to
the stars, this distribution is _not_ one of equilibrium, but implies
progressing concentration. While, just as we inferred that, according to
circumstances, the extent to which aggregation has been carried must
vary; so we find that, in fact, there are regular nebulae of all degrees
of resolvability, from those consisting of innumerable minute masses, to
those in which their numbers are smaller and the sizes greater, and to
those in which there are a few large bodies wort