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Spectroscopes
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A laboratory spectroscope. The light enters through a slit in the tube at the right, passes through the prism in the center, and is observed through the telescope at the left.
of light each element radiates. Such determinations have been made for all of the more than 80 chemical elements and for many compounds which are not broken up by the heat necessary to get them into the incandescent gaseous condition. In order to determine the character of an unknown substance its light is passed through a spectroscope and the result is compared with the known spectra of the different elements. If the substance is a simple element its spectrum will be precisely that of some element in the list of spectra. If the substance is a mixture of elements the corresponding spectra will be found. The fact that the substance is a mixture ordinarily makes very little difference. It is clear that the spectroscope may be used for making a chemical analysis. In the case of metals it is often the simplest and most certain method known. The spectroscope will prove the presence of sodium when the quantity is so minute that it would entirely escape ordinary chemical methods.
In order to use the customary methods of chemical analysis the substance under examination evidently must be actually in the possession of the chemist. On the other hand, when spectrum analysis is employed it is sufficient that the light shall reach the experimenter. If it travels a few feet in the laboratory, very well; if it comes across the 93,000,000 miles which separate us from the sun, also very well ; and if it comes from the enormously distant stars, still the method does not fail.
A little more than half a century ago a philosopher undertook to define the domains which it might be hoped human knowledge would sometime compass. He explained, among other things, how astronomers could measure the distances of the moon, planets, and sun. He showed how they could get the dimensions