motion the displacement that we observe represents only a part of the
actual movement of the star concerned. There are stars whose motion
carries them straight toward or straight away from the earth, and such
stars, of course, show no cross motion. But the vast majority are
traveling in paths inclined from a perpendicular to our line of sight.
Taken as a whole, the stars may be said to be flying about like the
molecules in a mass of gas. The discovery of the radial component in
the movements of the stars is due to the spectroscope. If a star is
approaching, its spectral lines are shifted toward the violet end of
the spectrum by an amount depending upon the velocity of approach; if
it is receding, the lines are correspondingly shifted toward the red
end. Spectroscopic observation, then, combined with micrometric
measurements of the cross motion, enables us to detect the real
movement of the star in space. Sometimes it happens that a star's
radial movement is periodically reversed; first it approaches, and
then it recedes. This indicates that it is revolving around a near-by
companion, which is often invisible, and superposed upon this motion
is that of the two stars concerned, which together may be approaching
or receding or traveling across the line of sight. Thus the
complications involved in the stellar motions are often exceedingly
great and puzzling.

Yet another source of complication exists in the movement of our own
star, the sun. There is no more difficult problem in astronomy than
that of disentangling the effects of the solar motion from those of
the motions of the other stars. But the problem, difficult as it is,
has been solved, and upon its solution depends our knowledge of the
speed and direction of the movement of the solar system through space,
for of course the sun carries its planets with it. One element of the
solution is found in the fact that, as a result of perspective, the