hall find the distance of its boundary so
great that millions of 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 thickness of the stars in space.
It does not seem probable that there are as many as 1,000,000,000 stars
within the sphere of 3300 light-years. Nor is it at all certain that
the light of the average star is equal to that of the sun. It is
impossible, in the present state of our knowledge, to assign any
definite value to this average. To do so is a problem similar to that
of assigning an average weight to each component of the animal
creation, from the microscopic insects which destroy our plants up to
the elephant. What we can say