there
are produced any of the planet-forming rings. But both of these
photographs apparently imply that, in some cases at any rate, the
portions of nebulous matter composing the rings take shape before they
reach the central mass. It looks as though these partially-formed annuli
must be prevented by their acquired motions from approaching even very
near to the still-irregular body they surround.

Be this as it may, however, and be the dimensions of the incipient
systems what they may (and it would seem to be a necessary implication
that they are vastly larger than our Solar System), the process remains
essentially the same. Practically demonstrated as this process now is,
we may say that the doctrine of nebular genesis passes from the region
of hypothesis into the region of established truth.

FOOTNOTES:

[Footnote 21: Of course there remains the question whether, before the
stage here recognized, there had already been produced a high
temperature by those collisions of celestial masses which reduced the
matter to a nebulous form. As suggested in _First Principles_ (Sec. 136 in
the edition of 1862, and Sec. 182 in subsequent editions), there must,
after there have been effected all those minor dissolutions which follow
evolutions, remain to be effected the dissolutions of the great bodies
in and on which the minor evolutions and dissolutions have taken place;
and it was argued that such dissolutions will be, at some time or other,
effected by those immense transformations of molar motion into molecular
motion, consequent on collisions: the argument being based on the
statement of Sir John Herschel, that in clusters of stars collisions
must inevitably occur. It may, however, be objected that though such a
result may be reasonably looked for in closely aggregated assemblages of
stars, it is difficult to conceive of its taking place throughout our
Sidereal System at large, the members of which, and their intervals, may
be roughly figured as pins-heads 50 miles apart. It would seem that
something like an eternity must elapse before, by ethereal resistance or
other cause, these can be brought into proximity great enough to make
collisions probable.]

[Footnote 22: The two sentences which, in the text, precede the
asterisk, I have introduced while these pages are standing in type:
being led to do so by the perusal of some notes kindly lent to me by
Prof. Dewar, containing the outline of a lecture he gave at the Royal
In