millions of years would be required for the light of the outer
stars to reach the centre of the system. In view of the fact that
this duration in time far exceeds what seems to be the possible
life duration of a star, so far as our knowledge of it can extend,
the mere fact that the sky does not glow with any such brightness
proves little or nothing as to the extent of the system.

We may, however, replace these purely negative considerations by
inquiring how much light we actually get from the invisible stars
of our system. Here we can make a definite statement. Mark out a
small circle in the sky 1 degree in diameter. The quantity of
light which we receive on a cloudless and moonless night from the
sky within this circle admits of actual determination. From the
measures so far available it would seem that, in the general
average, this quantity of light is not very different from that of
a star of the fifth magnitude. This is something very different
from a blaze of light. A star of the fifth magnitude is scarcely
more than plainly visible to ordinary vision. The area of the
whole sky is, in round numbers, about 50,000 times that of the
circle we have described. It follows that the total quantity of
light which we receive from all the stars is about equal to that
of 50,000 stars of the fifth magnitude--somewhat more than 1000 of
the first magnitude. This whole amount of light would have to be
multiplied by 90,000,000 to make a light equal to that of the sun.
It is, therefore, not at all necessary to consider how far the
system must extend in order that the heavens should blaze like the
sun. Adopting Lord Kelvin's hypothesis, we shall find that, in
order that we may receive from the stars the amount of light we
have designated, this system need not extend beyond some 5000
light-years. But this hypothesis probably overestimates the