this low state the light remains for twenty
minutes; then during three or four hours the gas is to be slowly
turned on again until it is full. In this condition the light will
remain for two days and a half, and then the same series of changes is
to recommence. This would be a very odd form of gas-lamp. There would
be periods of two days and a half during which it would remain at its
full; these would be separated by intervals of about seven hours, when
the gradual turning down and turning up again would be in progress.

The imaginary gas-lamp is exactly paralleled by a star Algol, in the
constellation of Perseus (Fig. 3), which goes through the series of
changes I have indicated. Ordinarily speaking, it is a bright star of
the second magnitude, and, whatever be the cause, the star performs
its variations with marvellous uniformity. In fact, Algol has always
arrested the attention of those who observed the heavens, and in early
times was looked on as the eye of a demon. There are many other stars
which also change their brilliancy. Most of them require much longer
periods than Algol, and sometimes a new star which nobody has ever
seen before will suddenly kindle into brilliancy. It is now known that
the bright star Algol is attended by a dark companion. This dark star
sometimes comes between Algol and the observer and cuts off the light.
Thus it is that the diminution of brightness is produced.

[Illustration: FIG. 3. PERSEUS AND ITS NEIGHBORING STARS INCLUDING
ALGOL.]

Double Stars.

Whenever you have a chance of looking at the heavens through a
telescope, you should ask to be shown what is called _a double star_.
There are many stars in the heavens which present no remarkable
appearance to the unaided eye, but which a good telescope at once
shows to be of quite a complex nature. These are what we call double
stars, in which two quite distinct stars are placed so close together
that the unaided eye is unable to separate them. Under the magnifying
power of the telescope, however, they are seen to be distinct. In
order to give some notion of what these objects are like, I shall
briefly describe three of them. The first lies in that best known
constellation, the Great Bear. If you look at his tail, which consists
of three stars, you will see that near the middle one of the three a
small star is situated; we call this little star Alcor, but it is the
brighter one near Alcor to which I specially call your attention.