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 the same. The fact that so many
more of these objects are seen around the galactic belt than in
the direction of its poles shows that, whatever extinction light
may suffer in going through the greatest distances, we see nearly
all that comes from stars not more distant than the Milky Way
itself.

Intimately connected with the subject we have discussed is the
question of the age of our system, if age it can be said to have.
In considering this question, the simplest hypothesis to suggest
itself is that the universe has existed forever in some such form