ill we could never prove that this
space is empty out to an infinite distance. Far outside of what we call
the universe might still exist other universes which we can never see.

It is a great encouragement to the astronomer that, although he cannot
yet set any exact boundary to this universe of ours, he is gathering
faint indications that it has a boundary, which his successors not many
generations hence may locate so that the astronomer shall include
creation itself within his mental grasp. It can be shown mathematically
that an infinitely extended system of stars would fill the heavens with
a blaze of light like that of the noonday sun. As no such effect is
produced, it may be concluded that the universe has a boundary. But
this does not enable us to locate the boundary, nor to say how many
stars may lie outside the farthest stretches of telescopic vision. Yet
by patient research we are slowly throwing light on these points and
reaching inferences which, not many years ago, would have seemed
forever beyond our powers.

Every one now knows that the Milky Way, that girdle of light which
spans the evening sky, is formed of clouds of stars too minute to be
seen by the unaided vision. It seems to form the base on which the
universe is built and to bind all the stars into a system. It comprises
by far the larger number of stars that the telescope has shown to
exist. Those we see with the naked eye are almost equally scattered
over the sky. But the number which the telescope shows us become more
and more condensed in the Milky Way as telescope power is increased.
The number of new stars brought out with our greatest power is vastly
greater in the Milky Way than in the rest of the sky, so that the
former contains a great majority of the stars. What is yet more
curious, spectroscopic research has shown that a particular kind of
stars, those formed of heated gas, are yet more condensed in the
central circle of this band; if they were visible to the naked eye, we
should see them encircling the heavens as a narrow girdle forming
perhaps the base of our whole system of stars. This arrangement of the
gaseous or vaporous stars is one of the most singular facts that modern
research has brought to light. It seems to show that these particular
stars form a system of their own; but how such a thing can be we are
still unable to see.

The question of the form and extent of the Milky Way thus becomes the
central one of stellar astrono