they are essentially different from the clusters of the Milky Way,
and cannot be regarded as forming an important part of the general
plan. In the case of the galaxy we have no such scattering, but
find the stars built, as it were, into this enormous ring, having
similar characteristics throughout nearly its whole extent, and
having within it a nearly uniform scattering of stars, with here
and there some collected into clusters. Such, to our limited
vision, now appears the universe as a whole.

We have already alluded to the conclusion that an absolutely
infinite system of stars would cause the entire heavens to be
filled with a blaze of light as bright as the sun. It is also true
that the attractive force within such a universe would be
infinitely great in some direction or another. But neither of
these considerations enables us to set a limit to the extent of
our system. In two remarkable papers by Lord Kelvin which have
recently appeared, the one being an address before the British
Association at its Glasgow meeting, in 1901, are given the results
of some numerical computations pertaining to this subject.
Granting that the stars are scattered promiscuously through space
with some approach to uniformity in thickness, and are of a known
degree of brilliancy, it is easy to compute how far out the system
must extend in order that, looking up at the sky, we shall see a
certain amount of light coming from the invisible stars. Granting
that, in the general average, each star is as bright as the sun,
and that their thickness is such that within a sphere of 3300
light-years there are 1,000,000,000 stars, if we inquire how far
out such a system must be continued in order that the sky shall
shine with even four per cent of the light of the sun, we shall
find the distance of its boundary so great that millions of