are distributed through space with some
approach to uniformity. We may state as a general conclusion, indicated
by several methods of making the estimate, that nearly all the stars
which we can see with our telescopes are contained within a sphere not
likely to be much more than 200,000,000 times the distance of the sun.

The inquiring reader may here ask another question. Granting that all
the stars we can see are contained within this limit, may there not be
any number of stars outside the limit which are invisible only because
they are too far away to be seen?

This question may be answered quite definitely if we grant that light
from the most distant stars meets with no obstruction in reaching us.
The most conclusive answer is afforded by the measure of starlight. If
the stars extended out indefinitely, then the number of those of each
order of magnitude would be nearly four times that of the magnitude
next brighter. For example, we should have nearly four times as many
stars of the sixth magnitude as of the fifth; nearly four times as many
of the seventh as of the sixth, and so on indefinitely. Now, it is
actually found that while this ratio of increase is true for the
brighter stars, it is not so for the fainter ones, and that the
increase in the number of the latter rapidly falls off when we make
counts of the fainter telescopic stars. In fact, it has long been known
that, were the universe infinite in extent, and the stars equally
scattered through all space, the whole heavens would blaze with the
light of countless millions of distant stars separately invisible even
with the telescope.

The only way in which this conclusion can be invalidated is by the
possibility that the light of the stars is in some way extinguished or
obstructed in its passage through space. A theory to this effect was
propounded by Struve nearly a century ago, but it has since been found
that the facts as he set them forth do not justify the conclusion,
which was, in fact, rather hypothetical. The theories of modern science
converge towards the view that, in the pure ether of space, no single
ray of light can ever be lost, no matter how far it may travel. But
there is another possible cause for the extinction of light. During the
last few years discoveries of dark and therefore invisible stars have
been made by means of the spectroscope with a success which would have
been quite incredible a very few years ago, and which, even to-day,