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 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