are, therefore, presented with what seems to be the most
extraordinary spectacle that the universe can offer, a ring of stars
spanning it, and including within its limits by far the great majority
of the stars within our system. We have in this spectacle another
example of the unity which seems to pervade the system. We might
imagine the latter so arranged as to show diversity to any extent. We
might have agglomerations of stars like those of the Milky Way situated
in some corner of the system, or at its centre, or scattered through it
here and there in every direction. But such is not the case. There are,
indeed, a few star-clusters scattered here and there through the
system; but they are essentially different from the clusters of the
Milky Way, and cannot be regarded as forming an important part of the
general plan. In the case of the galaxy we have no such scattering, but
find the stars built, as it were, into this enormous ring, having
similar characteristics throughout nearly its whole extent, and having
within it a nearly uniform scattering of stars, with here and there
some collected into clusters. Such, to our limited vision, now appears
the universe as a whole.

We have already alluded to the conclusion that an absolutely infinite
system of stars would cause the entire heavens to be filled with a
blaze of light as bright as the sun. It is also true that the
attractive force within such a universe would be infinitely great in
some direction or another. But neither of these considerations enables
us to set a limit to the extent of our system. In two remarkable papers
by Lord Kelvin which have recently appeared, the one being an address
before the British Association at its Glasgow meeting, in 1901, are
given the results of some numerical computations pertaining to this
subject. Granting that the stars are scattered promiscuously through
space with some approach to uniformity in thickness, and are of a known
degree of brilliancy, it is easy to compute how far out the system must
extend in order that, looking up at the sky, we shall see a certain
amount of light coming from the invisible stars. Granting that, in the
general average, each star is as bright as the sun, and that their
thickness is such that within a sphere of 3300 light-years there are
1,000,000,000 stars, if we inquire how far out such a system must be
continued in order that the sky shall shine with even four per cent of
the light of the sun, we s