estimated, by Mr. Gore and others, that only about 100
millions of stars are to be seen in the whole of the sky with the best
optical aids. This shows well the limited extent of the stellar system,
for the number is not really great. For instance, there are from fifteen
to sixteen times as many persons alive upon the earth at this moment!

Last of all, there appears to be strong photographic evidence that our
sidereal system is limited in extent. Two photographs taken by the late
Dr. Isaac Roberts of a region rich in stellar objects in the
constellation of Cygnus, clearly show what has been so eloquently called
the "darkness behind the stars." One of these photographs was taken in
1895, and the other in 1898. On both occasions the state of the
atmosphere was practically the same, and the sensitiveness of the films
was of the same degree. The exposure in the first case was only one
hour; in the second it was about two hours and a half. And yet both
photographs show _exactly the same stars, even down to the faintest_.
From this one would gather that the region in question, which is one of
the most thickly star-strewn in the Milky Way, is _penetrable right
through_ with the means at our command. Dr. Roberts himself in
commenting upon the matter drew attention to the fact, that many
astronomers seemed to have tacitly adopted the assumption that the stars
extend indefinitely through space.

From considerations such as these the foremost astronomical authorities
of our time consider themselves justified in believing that the
collection of stars around us is _finite_; and that although our best
telescopes may not yet be powerful enough to penetrate to the final
stars, still the rapid decrease in numbers as space is sounded with
increasing telescopic power, points strongly to the conclusion that the
boundaries of the stellar system may not lie very far beyond the
uttermost to which we can at present see.

Is it possible then to make an estimate of the extent of this stellar
system?

Whatever estimates we may attempt to form cannot however be regarded as
at all exact, for we know the actual distances of such a very few only
of the nearest of the stars. But our knowledge of the distances even of
these few, permits us to assume that the stars close around us may be
situated, on an average, at about eight light-years from each other; and
that this holds good of the stellar spaces, with the exception of the
encircling girdle of