posed, and we shall not be aware that we
really see two different spectra. But during the revolution of the two
bodies round their common centre of gravity there must periodically come
a time when one body is moving towards us and the other moving from us,
and consequently the lines in the spectrum of the former will be subject
to a minute, relative shift towards the violet end of the spectrum, and
those of the other to a minute shift towards the red. Those lines which
are common to the two spectra will therefore periodically become double.
A discovery of this sort was first made in 1889 by Professor Pickering
from photographs of the spectrum of Mizar, or z Ursa Majoris, the
larger component of the well-known double star in the tail of the Great
Bear. Certain of the lines were found to be double at intervals of
fifty-two days. The maximum separation of the two components of each
line corresponds to a relative velocity of one star as compared with the
other of about a hundred miles per second, but subsequent observations
have shown the case to be very complicated, either with a very eccentric
elliptic orbit or possibly owing to the presence of a third body. The
Harvard College photographs also showed periodic duplicity of lines in
the star b Aurigae, the period being remarkably short, only three
days and twenty-three hours and thirty-seven minutes. In 1891 Vogel
found, from photographs of the spectrum of Spica, the first magnitude
star in Virgo, that this star alternately recedes from and approaches to
the solar system, the period being four days. Certain other
"spectroscopic binaries" have since then been found, notably one
component of Castor, with a period of three days, found by M.
Belopolsky, and a star in the constellation Scorpio, with a period of
only thirty-four hours, detected on the Harvard spectrograms.

Quite recently Mr. H.F. Newall, at Cambridge, and Mr. Campbell, of the
Lick Observatory, have shown that a Aurigae, or Capella, consists
of a sun-like star and a Procyon-like star, revolving in 104 days.

At first sight there is something very startling in the idea of two suns
circling round each other, separated by an interval which, in comparison
with their diameters, is only a very small one. In the Algol system, for
instance, we have two bodies, one the size of our own sun and the other
slightly larger, moving round their common centre of gravity in less
than three days, and at a distance between their su