Page:Elementary Text-book of Physics (Anthony, 1897).djvu/469

§ 373] such as sulphate of quinine, uranium glass, and thallene, have the property, when illuminated by rays of short wave length, even by the invisible rays beyond the violet, of emitting light of longer wave length. Such substances are fluorescent. The light emitted by them, and the conditions favorable to their luminosity, have been studied by Stokes. It appears that the light emitted is of the same character, covering a considerable region of the spectrum, no matter what may be the incident light, provided this be such as to produce the effect at all. The light emitted is always of longer wave length than that which causes the luminosity.

There is another class of substances which, after being exposed to light, will glow for some time in the dark. These are phosphorescent. They must be carefully distinguished from such bodies as phosphorus and decaying wood, which glow in consequence of chemical action. Some phosphorescent substances, especially the calcium sulphides, glow for several hours after exposure. All fluorescent bodies are also phosphorescent, but the time during which they remain luminous after the exciting light is removed, is so short that it can generally be detected only by special devices.

373. Anomalous Dispersion.—As has been already stated, there is a class of bodies which show a selective absorption at their surfaces. The light reflected from such bodies is complementary to the light which they can transmit. Kundt, following up isolated observations of other physicists, has shown that all such bodies give rise to an anomalous dispersion; that is, the order of the colors in the spectrum formed by a prism of one of these substances is not the same as their order in the diffraction spectrum or in the spectrum formed by prisms of substances which do not show selective absorption at their surfaces. Solid fuchsin, when viewed by reflected light, appears green. In solution, when viewed by transmitted light, it appears red. Christiansen allowed light to pass through a prism formed of two glass plates making a small angle with each other, and containing a solution of fuchsin in alcohol. He found that the green was almost totally wanting in