es
in fact revolve round a dark, and therefore invisible, companion. The
spectroscope, in fact, makes known to us the presence of many stars
which no telescope could reveal.

Thus the floor of heaven is not only "thick inlaid with patines of
bright gold," but studded also with extinct stars, once probably as
brilliant as our own Sun, but now dead and cold, as Helmholtz tells us
that our Sun itself will be some seventeen millions of years hence.

Such dark bodies cannot of course be seen, and their existence, though
we cannot doubt it, is a matter of calculation. In one case, however,
the conclusion has received a most interesting confirmation. The
movements of Sirius led mathematicians to conclude that it had also a
mighty and massive neighbour, the relative position of which they
calculated, though no such body had ever been seen. In February 1862,
however, the Messrs. Alvan Clark of Cambridgeport were completing their
18-inch glass for the Chicago Observatory. "'Why, father,'" exclaimed
the younger Clark, "'the star has a companion.' The father looked, and
there was a faint star due east from the bright one, and distant about
ten seconds. This was exactly the predicted direction for that time,
though the discoverers knew nothing of it. As the news went round the
world many observers turned their attention to Sirius; and it was then
found that, though it had never before been noticed, the companion was
really shown under favourable circumstances by any powerful telescope.
It is, in fact, one-half of the size of Sirius, though only 1/10000th of
the brightness."[72]

Stars are, we know, of different magnitudes and different degrees of
glory. They are also of different colours. Most, indeed, are white, but
some reddish, some ruddy, some intensely red; others, but fewer, green,
blue, or violet. It is possible that the comparative rarity of these
colours is due to the fact that our atmosphere especially absorbs green
and blue, and it is remarkable that almost all of the green, blue, or
violet stars are one of the pairs of a Double Star, and in every case
the smaller one of the two, the larger being red, orange, or yellow. One
of the most exquisite of these is [Greek: beta] Cygni, a Double Star, the
larger one being golden yellow, the smaller light blue. With a telescope
the effect is very beautiful, but it must be magnificent if one could
only see it from a lesser distance.

Double Stars occur in considerable numbers. I