alue.
Spectroscopic investigations of stellar movements may confidently be
expected to play a leading part in the unravelment of the vast and
complex relations which we can dimly detect as prevailing among the
innumerable orbs of the sidereal world; for it supplements the means
which we possess of measuring by direct observation movements transverse
to the line of sight, and thus completes our knowledge of the courses
and velocities of stars at ascertained distances, while supplying for
all a valuable index to the amount of perspective foreshortening of
apparent movement. Thus some, even if an imperfect, knowledge may at
length be gained of the revolutions of the stars--of the systems they
unite to form, of the paths they respectively pursue, and of the forces
under the compulsion of which they travel.

The applicability of the method to determining the orbital motions of
double stars was pointed out by Fox Talbot in 1871;[1439] but its use
for their discovery revealed itself spontaneously through the Harvard
College photographs. In "spectrograms" of Zeta Ursae Majoris
(Mizar), taken in 1887, and again in 1889, the K line was seen to be
double; while on other plates it appeared single. A careful study of
Miss A. C. Maury of a series of seventy impressions indicated for the
doubling a period of fifty-two days, and showed it to affect all the
lines in the spectrum.[1440] The only available, and no doubt the true,
explanation of the phenomenon was that two similar and nearly equal
stars are here merged into one telescopically indivisible; their
combined light giving a single or double spectrum, according as their
orbital velocities are directed across or along our line of sight. The
movements of a revolving pair of stars must always be opposite in sense,
and proportionately equal in amount. That is, they at all times travel
with speeds in the inverse ratio of their masses. Hence, unless the
plane of their orbits be perpendicular to a plane passing through the
eye, there must be two opposite points where their velocities in the
line of sight reach a maximum, and two diametrically opposite points
where they touch zero. The lines in their common spectrum would thus
appear alternately double and single twice in the course of each
revolution. To that of Mizar, at first supposed to need 104 days for its
completion, a period of only twenty days fourteen hours was finally
assigned by Vogel.[1441] Anomalous spectral effects, probab