c binaries appear to be upon a smaller scale
than the telescopic ones. Some are, indeed, comparatively speaking,
quite small. For instance, the component stars forming [b] Aurigae are
about eight million miles apart, while in [z] Geminorum, the distance
between the bodies is only a little more than a million miles.

Spectroscopic binaries are probably very numerous. Professor W.W.
Campbell, Director of the Lick Observatory, estimates, for instance,
that, out of about every half-a-dozen stars, one is a spectroscopic
binary.

It is only in the case of binary systems that we can discover the masses
of stars at all. These are ascertained from their movements with regard
to each other under the influence of their mutual gravitative
attractions. In the case of simple stars we have clearly nothing of the
kind to judge by; though, if we can obtain a parallax, we may hazard a
guess from their brightness.

Binary stars were incidentally discovered by Sir William Herschel. In
his researches to get a stellar parallax he had selected a number of
double stars for test purposes, on the assumption that, if one of such a
pair were much nearer than the other, it might show a displacement with
regard to its neighbour as a direct consequence of the earth's orbital
movement around the sun. He, however, failed entirely to obtain any
parallaxes, the triumph in this being, as we have seen, reserved for
Bessel. But in some of the double stars which he had selected, he found
certain alterations in the relative positions of the bodies, which
plainly were not a consequence of the earth's motion, but showed rather
that there was an actual circling movement of the bodies themselves
under their mutual attractions. It is to be noted that the existence of
such connected pairs had been foretold as probable by the Rev. John
Michell, who lived a short time before Herschel.

The researches into binary systems--both those which can be seen with
the eye and those which can be observed by means of the spectroscope,
ought to impress upon us very forcibly the wide sway of the law of
gravitation.

Of star clusters about 100 are known, and such systems often contain
several thousand stars. They usually cover an area of sky somewhat
smaller than the moon appears to fill. In most clusters the stars are
very faint, and, as a rule, are between the twelfth and sixteenth
magnitudes. It is difficult to say whether these are actually small
bodies, or whether their