in a similar time it
regains its former brilliancy.

The conclusion that has been arrived at in regard to the cause of the
variation of these stars is, that in each case the diminution of light
is due to the existence of dark bodies, probably planets, which revolve
round the central star.

This hypothesis was confirmed by Professor Vogel about 1889 by means of
spectroscopic results.

Another interesting fact about stars is that they shine with various
colours. The colours of stars are as various as the colours of the
rainbow, and range through the whole spectrum, of red, orange, yellow,
green, blue, indigo, violet, and white. What is more remarkable is the
fact that the colours of the stars seem to change through great periods
of time. If we turn to ancient records we learn that Sirius was red
then, but is now green, while Capella was also red, but is now pale
blue.

_Double and Multiple Stars._--Many stars when looked at through powerful
telescopes are found to be double, triple, quadruple, and even multiple,
although when looked at by the naked eye, they seem to be single in
appearance.

An example of a double star is to be found in the constellation of Lyra.
A moderate telescope reveals this as a double star, while a still more
powerful telescope reveals the strange fact that each apparently single
star which forms the double is itself double, so that we have in this
constellation a system of four stars, in which each pair revolves round
a point situated between them.

Several thousand double stars are known altogether, while the motions of
several hundreds of them have been detected with powerful telescopes.
Some of the double stars are as follows--Zeta Hercules, Eta Coronae
Borealis, Gamma Coronae Borealis, Beta Cygni, Alpha Centauri.

The colours of some of the double stars are very beautiful. Some are
yellow and blue; others, yellow and purple, while others are orange and
green. Some of the double stars are only optical doubles, that is to
say, they apparently seem close together, while as a matter of fact they
are immense distances from each other, the apparent doubleness being due
to the fact that they are more or less in the same line of vision. Real
double stars, where the component stars are situated close together, are
known as physical doubles, to distinguish them from the optical doubles.

_Binary Stars._--Another class of double stars are known as Binary
Stars. This class of stars is composed o