Page:Elementary Text-book of Physics (Anthony, 1897).djvu/462

448 of it may, by moving it toward one or the other, be made to appear alike, and the translucent portion almost invisible. The light transmitted through this portion in one direction then equals that transmitted in the opposite direction; that is, the two surfaces are equally illuminated.

365. Transmission and Absorption of Radiations.—It is a familiar fact that colored glass transmits light of certain colors only, and the inference is easy that the other colors are absorbed by the glass. It is only necessary to form a spectrum, and place the colored glass in the path of the light either before or after the separation of the colors, to show which colors are transmitted, and which absorbed.

By the use of the thermopile or bolometer, both of which are sensitive to radiations of all periods of vibration, it is found that some bodies are apparently perfectly transparent to light, and opaque to the obscure radiations. Clear, white glass is opaque to a large portion of the obscure rays of long wave length. Water and solution of alum are still more opaque to these rays, and pure ice transmits almost none of the radiations of which the wave lengths are longer than those of the visible red. Eock salt transmits well both the luminous and the non-luminous radiations.

On the other hand, some substances apparently opaque are transparent to radiations of long wave length. A plate of glass or rock salt rendered opaque to light by smoking it over a lamp is still as transparent as before to the radiations of longer wave length. Selenium is opaque to light, but transparent to the radiations of longer wave length. This fact explains the change of its electrical resistance by light, but not by non-luminous rays. Carbon disulphide, like rock salt, transmits nearly equally the luminous and non-luminous rays; but if iodine be dissolved in it, it will at first cut off the luminous rays of shorter wave length, and as the solution becomes more and more concentrated the absorption extends down the spectrum to the red, and finally all light is extinguished, and the solution to the eye becomes opaque. The radiations of which the wave lengths are longer than those of the red still pass