the solar system, consisting of the sun, and the planets,
with their satellites, the comets, and a host of smaller bodies, formed
merely a little island group in the universe. In the economy of this
tiny cosmical island the law of gravitation reigns supreme; before
Herschel's discovery we never could have known whether that law was not
merely a piece of local legislation, specially contrived for the
exigencies of our particular system. This discovery gave us the
knowledge which we could have gained from no other source. From the
binary stars came a whisper across the vast abyss of space. That whisper
told us that the law of gravitation was not peculiar to the solar
system. It told us the law extended to the distant shores of the abyss
in which our island is situated. It gives us grounds for believing that
the law of gravitation is obeyed throughout the length, breadth, and
depth of the entire visible universe.

One of the finest binary stars is that known as Castor, the brighter of
the two principal stars in the constellation of Gemini. The position of
Castor on the heavens is indicated in Fig. 86, page 418. Viewed by the
unaided eye, Castor resembles a single star; but with a moderately good
telescope it is found that what seems to be one star is really two
separate stars, one of which is of the third magnitude, while the other
is somewhat less. The angular distance of these two stars in the heavens
is not so great as the angle subtended by a line an inch long viewed at
a distance of half a mile. Castor is one of the double stars in which
the components have been observed to possess a motion of revolution. The
movement is, however, extremely slow, and the lapse of centuries will be
required before a revolution is completely effected.

A beautiful double star can be readily identified in the constellation
of Ursa Major (_see_ Fig. 80, page 410). It is known as Mizar, and is
the middle star (z) of the three which form the tail. In the close
neighbourhood of Mizar is the small star Alcor, which can be readily
seen with the unaided eye; but when we speak of Mizar as a double star,
it is not to be understood that Alcor is one of the components of the
double. Under the magnifying power of the telescope Alcor is seen to be
transferred a long way from Mizar, while Mizar itself is split up into
two suns close together. These components are of the second and the
fourth magnitudes respectively, and as the apparent distance is ne