. I dare say you have often noticed the red and the
green lights that are used on railways in the signal lamps. Imagine
one of those red and one of those green lights away far up in the sky
and placed close together, then you would have some idea of the
appearance that a colored double star presents, though, perhaps, I
should add that the hues in the heavenly bodies are not so vividly
different as are those which our railway people find necessary. There
is a particularly beautiful double star of this kind in the
constellation of the Swan. You could make an imitation of it by boring
two holes, with a red-hot needle, in a piece of card, and then
covering one of these holes with a small bit of the topaz-colored
gelatine with which Christmas crackers are made. The other star is to
be similarly colored with blue gelatine. A slide made on this
principle placed in the lantern gives a very good representation of
these two stars on the screen. There are many other colored doubles
besides this one; and, indeed, it is noteworthy that we hardly ever
find a blue or a green star by itself in the sky; it is always as a
member of one of these pairs.

How We Find What the Stars are Made of.

Here is a piece of stone. If I wanted to know what it was composed of,
I should ask a chemist to tell me. He would take it into his
laboratory, and first crush it into powder, and then, with his test
tubes, and with the liquids which his bottles contain, and his
weighing scales, and other apparatus, he would tell all about it;
there is so much of this, and so much of that, and plenty of this, and
none at all of that. But now, suppose you ask this chemist to tell you
what the sun is made of, or one of the stars. Of course, you have not
a sample of it to give him; how, then, can he possibly find out
anything about it? Well, he can tell you something, and this is the
wonderful discovery that I want to explain to you. We now put down the
gas, and I kindle a brilliant red light. Perhaps some of those whom I
see before me have occasionally ventured on the somewhat dangerous
practice of making fire-works. If there is any boy here who has ever
constructed sky-rockets, and put the little balls into the top which
are to burn with such vivid colors when the explosion takes place, he
will know that the substance which tinged that fire red must have been
strontium. He will recognize it by the color; because strontium gives
a red light which nothing else will