wn to the
astronomer to learn anything about the universe as a whole? We may
commence by answering this question in a somewhat comprehensive way. It
is possible only because the universe, vast though it is, shows certain
characteristics of a unified and bounded whole. It is not a chaos, it
is not even a collection of things, each of which came into existence
in its own separate way. 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 near the Milky Way, we
shall find more stars in both of them than elsewhere; if we take them
in the region anywhere around the poles of the Milky Way, we shall find
fewer stars, but they will be equally numerous in each of the two
regions. We infer from this that whatever cause determined the number
of the stars in space was of the same nature in every tw