The claim
has therefore been made that there is no proof of the system
extending out any farther in the equatorial than in the polar
direction.
The consideration of this objection requires a closer inquiry as
to what we are to understand by the form of our system. We have
already pointed out the impossibility of assigning any boundary
beyond which we can say that nothing exists. And even as regards a
boundary of our stellar system, it is impossible for us to assign
any exact limit beyond which no star is visible to us. The analogy
of collections of stars seen in various parts of the heavens leads
us to suppose that there may be no well-defined form to our
system, but that, as we go out farther and farther, we shall see
occasional scattered stars to, possibly, an indefinite distance.
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.
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