with a fair approximation to confidence
is that, if we could fly out in any direction to a distance of 20,000,
perhaps even of 10,000, light-years, we should find that we had left a
large fraction of our system behind us. We should see its boundary in
the direction in which we had travelled much more certainly than we see
it from our stand-point.

We should not dismiss this branch of the subject without saying that
considerations are frequently adduced by eminent authorities which tend
to impair our confidence in almost any conclusion as to the limits of
the stellar system. The main argument is based on the possibility that
light is extinguished in its passage through space; that beyond a
certain distance we cannot see a star, however bright, because its
light is entirely lost before reaching us. That there could be any loss
of light in passing through an absolute vacuum of any extent cannot be
admitted by the physicist of to-day without impairing what he considers
the fundamental principles of the vibration of light. But the
possibility that the celestial spaces are pervaded by matter which
might obstruct the passage of light is to be considered. We know that
minute meteoric particles are flying through our system in such numbers
that the earth encounters several millions of them every day, which
appear to us in the familiar phenomena of shooting-stars. If such
particles are scattered through all space, they must ultimately
obstruct the passage of light. We know little of the size of these
bodies, but, from the amount of energy contained in their light as they
are consumed in the passage through our atmosphere, it does not seem at
all likely that they are larger than grains of sand or, perhaps, minute
pebbles. They are probably vastly more numerous in the vicinity of the
sun than in the interstellar spaces, since they would naturally tend to
be collected by the sun's attraction. In fact there are some reasons
for believing that most of these bodies are the debris of comets; and
the latter are now known to belong to the solar system, and not to the
universe at large.

But whatever view we take of these possibilities, they cannot
invalidate our conclusion as to the general structure of the stellar
system as we know it. Were meteors so numerous as to cut off a large
fraction of the light from the more distant stars, we should see no
Milky Way, but the apparent thickness of the stars in every direction
would be nearly th