Page:Elementary Text-book of Physics (Anthony, 1897).djvu/463

§ 368] freely. Black vulcanite seems perfectly opaque, yet it also transmits radiations of long wave length. If the radiations of the electric lamp be concentrated by means of a lens, and a sheet of black vulcanite placed between the lamp and the lens, bodies may be still heated in the focus.

366. Colors of Bodies.—Bodies become visible by the light which comes from them to the eye, and bodies which are not self-luminous must become visible by sending to the eye some portion of the light that falls on them. Of the light which falls on a body, part is reflected from the surface; the remainder which enters the body is, in general, partly absorbed, and the unabsorbed portion either goes on through the body, or is turned back by reflection at a greater or less depth within the body, and mingles with the light reflected from the surface.

In general the surface reflection is small in amount, and the different colors are reflected almost in the proportion in which they exist in the incident light. Much the larger portion of the light by which a body becomes visible is turned back after penetrating a short distance beneath the surface, and contains those colors which the substance does not absorb. This determines the color of the object. In many instances there is a selective reflection from the surface (§ 373). For example, the light reflected from gold-leaf is yellow, while that which it transmits is green. 367. Absorption by Gases.—If a pure spectrum be formed from the white light of the electric lamp, and sodium vapor, obtained by heating a bit of sodium or a bead of common salt in the Bunsen flame, be placed in the path of the beam, two narrow, sharply defined dark lines will be seen to cross the spectrum in the exact position that would be occupied by the yellow lines constituting the spectrum of sodium vapor. Gases in general have an effect similar to that of the vapor of sodium; that is, they absorb from the light which passes through them distinct radiations corresponding to definite wave lengths, which are always the same as those which would be emitted by the gas were it rendered incandescent.

368. Spectrum Analysis.—If the light of a lamp or of any