apple-green and cherry-red,
and so on. In the Southern Hemisphere there is a cluster containing so
many stars of brilliant colours that Sir John Herschel named it 'the
Jewelled Cluster.'

I expect most of you have seen an advertisement of Pear's Soap, in which
you are asked to stare at some red letters, and then look away to some
white surface, such as a ceiling, when you will see the same letters in
green. This is because green is the complementary or contrasting colour
to red, and the same thing is the case with blue and yellow. When any
one colour of either of these pairs is seen, it tends to make the other
appear by reaction, and if the eye gazed hard at blue instead of red, it
would next see yellow, and not green. Now, many people to whom this
curious fact is known argue that perhaps the colours of the double
stars are not real, but the effect of contrast only; for instance, they
say a red star near a companion white one would tend to make the
companion appear green, and so, of course, it would. But this does not
account for the star colours, which are really inherent in the stars
themselves, as may be proved by cutting off the light of one star, and
looking only at the other, when its colour still appears unchanged.
Another argument equally strong against the contrast theory is that the
colours of stars in pairs are by no means always those which would
appear if the effect was only due to complementary colours. It is not
always blue and yellow or red and green pairs that we see, though these
are frequent, but many others of various kinds, such as copper and blue,
and ruddy and blue.

We have therefore come to the conclusion that there are in this
astonishing universe numbers of gloriously coloured suns, some of which
apparently lie close together. What follows? Why, we want to know, of
course, if these stars are really pairs connected with each other, or if
they only appear so by being in the same line of sight, though one is
infinitely more distant than the other. And that question also has been
answered. There are now known thousands of cases in which stars,
hitherto regarded as single, have been separated into two, or even more,
by the use of a telescope. Of these thousands, some hundreds have been
carefully investigated, and the result is that, though there are
undoubtedly some in which the connexion is merely accidental, yet in by
far the greater number of cases the two stars thus seen together have
really