arly
three times as great as in Castor, they are observed with facility even
in a small telescope. This is, indeed, the best double star in the
heavens for the beginner to commence his observations upon. We cannot,
however, assert that Mizar is a binary, inasmuch as observations have
not yet established the existence of a motion of revolution. Still less
are we able to say whether Alcor is also a member of the same group, or
whether it may not merely be a star which happens to fall nearly in the
line of vision. Recent spectroscopic observations have shown that the
larger component of Mizar is itself a double, consisting of a pair of
suns so close together that there is not the slightest possibility of
their ever being seen separately by the most powerful telescope in the
world.

A pleasing class of double stars is that in which we have the remarkable
phenomenon of colours, differing in a striking degree from the colours
of ordinary stars. Among the latter we find, in the great majority of
cases, no very characteristic hue; some are, however, more or less
tinged with red, some are decidedly ruddy, and some are intensely red.
Stars of a bluish or greenish colour are much more rare,[37] and when a
star of this character does occur, it is almost invariably as one of a
pair which form a double. The other star of the double is sometimes of
the same hue, but more usually it is yellow or ruddy.

One of the loveliest of these objects, which lies within reach of
telescopes of very moderate pretensions, is that found in the
constellation of the Swan, and known as b Cygni (Fig. 91). This
exquisite object is composed of two stars. The larger, about the third
magnitude, is of a golden-yellow, or topaz, colour; the smaller, of the
sixth magnitude, is of a light blue. These colours are nearly
complementary, but still there can be no doubt that the effect is not
merely one of contrast. That these two stars are both tinged with the
hues we have stated can be shown by hiding each in succession behind a
bar placed in the field of view. It has also been confirmed in a very
striking manner by spectroscopic investigation; for we see that the blue
star has experienced a special absorption of the red rays, while the
more ruddy light of the other star has arisen from the absorption of the
blue rays. The contrast of the colours in this object can often be very
effectively seen by putting the eye-piece out of focus. The discs thus
produced show