Fig. 404.—A rainbow formed by a beam of light striking a flask of water.
Fig. 405.—The course of a beam of light within a drop of water.
407. The Spectroscope and Its Uses.—The spectroscope (Fig. 406) is an instrument for observing spectra. It consists of a prism, a slit, and a convex lens T for focusing an image of the slit accurately upon a screen (Fig. 407) where the spectrum is observed through the eyepiece E.
Fig. 406.—The spectroscope.
(A) A Bunsen flame is placed in front of the slit and a heated platinum wire which has been dipped in common salt or some sodium compound placed in the Bunsen flame; the latter becomes yellow and a vivid yellow line is observed on the screen in the spectroscope. Other substances, as barium and strontium salts, when heated to incandescence in the Bunsen flame, give characteristic bright lines. In fact each element has been found to have its own characteristic set of colored lines. This fact is made use of in spectrum analysis, by which the presence of certain elements in a substance can be definitely proved upon the appearance of its particular lines in the spectrum.
Fig. 407.—Diagram of a spectroscope.
