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 field for inquiry. The difficulty was, of course, to get the light of one star separated from all the rest, because the light of one star is very faint and feeble to cast a spectrum at all. Yet by infinite patience difficulties were overcome. One star alone was allowed to throw its light into the telescope; the light passed through a prism, and showed a faint band of many colours, with the expected little black lines cutting across it more or less thickly. Examinations have thus been made of hundreds of stars. In the course of them some substances as yet unknown to us on earth have been encountered, and in some stars one element--hydrogen--is much stronger than in others; but, on the whole, speaking broadly, it has been satisfactorily shown that the stars are made on the same principles as our own sun, so that the reasoning of astronomers which had argued them to be suns was proved.