The truth probably is that, as in ascending a mountain, we find
the trees, which may be very dense at its base, thin out gradually
as we approach the summit, where there may be few or none, so we
might find the stars to thin out could we fly to the distant
regions of space. The practical question is whether, in such a
flight, we should find this sooner by going in the direction of
the axis of our system than by directing our course towards the
Milky Way. If a point is at length reached beyond which there are
but few scattered stars, such a point would, for us, mark the
boundary of our system. From this point of view the answer does
not seem to admit of doubt. If, going in every direction, we mark
the point, if any, at which the great mass of the stars are seen
behind us, the totality of all these points will lie on a surface
of the general form that Herschel supposed.

There is still another direct indication of the finitude of our
stellar system upon which we have not touched. If this system
extended out without limit in any direction whatever, it is shown
by a geometric process which it is not necessary to explain in the
present connection, but which is of the character of mathematical
demonstration, that the heavens would, in every direction where
this was true, blaze with the light of the noonday sun. This would
be very different from the blue-black sky which we actually see on
a clear night, and which, with a reservation that we shall
consider hereafter, shows that, how far so-ever our stellar
system may extend, it is not infinite. Beyond this negative
conclusion the fact does not teach us much. Vast, indeed, is the
distance to which the system might extend without the sky
appearing much brighter than it is, and we must have recourse to
other considerations in seeking for indications of a boundary, or