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.
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