If it were, there would be nothing in common between two widely
separate regions of the universe. But, as a matter of fact,
science shows unity in the whole structure, and diversity only in
details. The Milky Way itself will be seen by the most ordinary
observer to form a single structure. This structure is, in some
sort, the foundation on which the universe is built. It is a
girdle which seems to span the whole of creation, so far as our
telescopes have yet enabled us to determine what creation is; and
yet it has elements of similarity in all its parts. What has yet
more significance, it is in some respects unlike those parts of
the universe which lie without it, and even unlike those which lie
in that central region within it where our system is now situated.
The minute stars, individually far beyond the limit of visibility
to the naked eye, which form its cloudlike agglomerations, are
found to be mostly bluer in color, from one extreme to the other,
than the general average of the stars which make up the rest of
the universe.

In the preceding essay on the structure of the universe, we have
pointed out several features of the universe showing the unity of
the whole. We shall now bring together these and other features
with a view of showing their relation to the question of the
extent of the universe.

The Milky Way being in a certain sense the foundation on which the
whole system is constructed, we have first to notice the symmetry
of the whole. This is seen in the fact that a certain resemblance
is found in any two opposite regions of the sky, no matter where
we choose them. If we take them in the Milky Way, the stars are
more numerous than elsewhere; if we take opposite regions in or