ected pairs of stars, since they cannot be seen
separately by means of any telescope, no matter how large, are known as
"spectroscopic binaries."

In observations of spectroscopic binaries we do not always get a double
spectrum. Indeed, if one of the components be below a certain
magnitude, its spectrum will not appear at all; and so we are left in
the strange uncertainty as to whether this component is merely faint or
actually dark. It is, however, from the shiftings of the lines in the
spectrum of the other component that we see that an orbital movement is
going on, and are thus enabled to conclude that two bodies are here
connected into a system, although one of these bodies resolutely refuses
directly to reveal itself even to the all-conquering spectroscope.

Mizar, that star in the handle of the Plough to which we have already
drawn attention, will be found with a small telescope to be a fine
double, one of the components being white and the other greenish.
Actually, however, as the American astronomer, Professor F.R. Moulton,
points out, these stars are so far from each other that if we could be
transferred to one of them we should see the other merely as an ordinary
bright star. The spectroscope shows that the brighter of these stars is
again a binary system of two huge suns, the components revolving around
each other in a period of about twenty days. This discovery made by
Professor E.C. Pickering, the _first_ of the kind by means of the
spectroscope, was announced in 1889 from the Harvard Observatory in the
United States.

A star close to Vega, known as [e] (Epsilon) Lyrae (see Plate XIX., p.
292), is a double, the components of which may be seen separately with
the naked eye by persons with very keen eyesight. If this star, however,
be viewed with the telescope, the two companions will be seen far apart;
and it will be noticed that each of them is again a double.

By means of the spectroscope Capella is shown to be really composed of
two stars (one about twice as bright as the other) situated very close
together and forming a binary system. Sirius is also a binary system;
but it is what is called a "visual" one, for its component stars may be
_seen_ separately in very large telescopes. Its double, or rather
binary, nature, was discovered in 1862 by the celebrated optician Alvan
G. Clark, while in the act of testing the 18-inch refracting telescope,
then just constructed by his firm, and now at the Dearborn