ch it is seen is called a
spectroscope. But this in itself could tell us little; the message it
brings lies in the fact that when it has passed through the telescope,
so that it is magnified, it is crossed by hundreds of minute black
lines, not placed evenly at all, but scattered up and down. There may be
two so close together that they look like one, and then three far apart,
and then some more at different distances. When this remarkable
appearance was examined carefully it was found that in sunlight the
lines that appeared were always exactly the same, in the same places,
and this seemed so curious that men began to seek for an explanation.

Someone thought of an experiment which might teach us something about
the matter, and instead of letting sunlight fall on the prism, he made
an artificial light by burning some stuff called sodium, and then
allowed the band of coloured light to pass through the telescope; when
he examined the spectrum that resulted, he found that, though numbers of
lines to be found in the sun's spectrum were missing, there were a few
lines here exactly matching a few of the lines in the sun's spectrum;
and this could not be the result of chance only, for the lines are so
mathematically exact, and are in themselves so peculiarly distributed,
that it could only mean that they were due to the same cause. What could
this signify, then, but that away up there in the sun, among other
things, stuff called sodium, very well known to chemists on earth, is
burning? After this many other substances were heated white-hot so as to
give out light, in order to discover if the lines to be seen in their
spectra were also to be found in the sun's spectrum. One of these was
iron, and, astonishing to say, all the many little thread-like lines
that appeared in its spectrum were reproduced to a hair's-breadth, among
others, in the sun's spectrum. So we have found out beyond all
possibility of doubt some of the materials of which the sun is made. We
know that iron, sodium, hydrogen, and numerous other substances and
elements, are all burning away there in a terrific furnace, to which any
furnace we have on earth is but as the flicker of a match.

It was not, of course, much use applying this method to the planets, for
we know that the light which comes from them to us is only reflected
sunlight, and this, indeed, was proved by means of the spectroscope. But
the stars shine by their own light, and this opened up a wide