Moreover, we can thus only
determine the motions at right angles to the line of sight, the motion
towards us or from us being entirely insensible in this way. Then came
the discovery of the change in the spectrum when a body was in motion,
but still this change was so small that visual observations of it proved
of but little value. Attaching a carefully constructed spectroscope to
one of the great telescopes of the world, photographing the spectrum of
a star, and measuring it with the greatest care, provided a tool of
wonderful efficiency. The motion, which sometimes amounts to several
hundreds of miles a second could thus be measured to within a fraction
of a mile. The discovery that the motion was variable, owing to the
star's revolving around a great dark planet sometimes larger than the
star, added greatly not only to the interest of these researches, but
also to the labor involved. Instead of a single measure for each star,
in the case of the so-called spectroscopic binaries, we must make enough
measures to determine the dimensions of the orbit, its form and the
period of revolution.
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