st of the stars in the opposite
direction, just as passing through a country on a railway we see the
houses on the right and on the left being left behind us. It is clear
enough that the apparent motion will be more rapid the nearer the
object. We may therefore form some idea of the distance of the stars
when we know the amount of the motion. It is found that in the great
mass of stars of the sixth magnitude, the smallest visible to the naked
eye, the motion is about three seconds per century. As a measure thus
stated does not convey an accurate conception of magnitude to one not
practised in the subject, I would say that in the heavens, to the
ordinary eye, a pair of stars will appear single unless they are
separated by a distance of 150 or 200 seconds. Let us, then, imagine
ourselves looking at a star of the sixth magnitude, which is at rest
while we are carried past it with the motion of six to eight miles per
second which I have described. Mark its position in the heavens as we
see it to-day; then let its position again be marked five thousand
years hence. A good eye will just be able to perceive that there are
two stars marked instead of one. The two would be so close together
that no distinct space between them could be perceived by unaided
vision. It is due to the magnifying power of the telescope, enlarging
such small apparent distances, that the motion has been determined in
so small a period as the one hundred and fifty years during which
accurate observations of the stars have been made.

The motion just described has been fairly well determined for what,
astronomically speaking, are the brighter stars; that is to say, those
visible to the naked eye. But how is it with the millions of faint
telescopic stars, especially those which form the cloud masses of the
Milky Way? The distance of these stars is undoubtedly greater, and the
apparent motion is therefore smaller. Accurate observations upon such
stars have been commenced only recently, so that we have not yet had
time to determine the amount of the motion. But the indication seems to
be that it will prove quite a measurable quantity and that before the
twentieth century has elapsed, it will be determined for very much
smaller stars than those which have heretofore been studied. A
photographic chart of the whole heavens is now being constructed by an
association of observatories in some of the leading countries of the
world. I cannot say all the leading coun