In 1802 Dr. Wollaston, in repeating Newton’s experiments, admitted the beam of light through a very narrow slit, instead of a round hole, and noticed that the spectrum, as spread out in its colors, was not a continuous shading from one color into another, but he found black lines crossing the spectrum. These black lines were, in 1814, carefully mapped by a German optician, named Fraunhofer, and were found by him to be 576 in number. The next step toward the spectroscope was made by Simms, an optician, in 1830, who placed a lens in front of the prism so that the slit was in the focus of the lens, and the light passing through the slit first passed through the lens, and then through the prism. This lens was called the “Collimating” lens. With these preliminary steps of development, Prof. Kirchhoff began in 1859 his great work of mapping the solar spectrum, and he, in connection with Prof. Bunsen, found several thousand of the dark lines in the spectrum, and laid the foundation of _spectrum-analysis_, or the determination of the nature of substances from the spectra cast by them when in an incandescent state. [Illustration: FIG. 199.--KIRCHHOFF’S FOUR-PRISM SPECTROSCOPE.] The form of Kirchhoff’s spectroscope is given in Fig. 199. The slit forming slide is seen on the far end of the tube A, and is shown in enlarged detached view on the right. The collimating lens is contained in the tube A. The beam of light entering the slit at the far end of the tube A, passes through the lens in that tube, and then passes successively through the four triangular prisms on the table, and is successively bent by these and thrown in the form of a spectrum into the telescopic tube B, and is seen by the eye at the remote end of said tube B. The greater the number of prisms the wider is the dispersion of the rays and the longer is the spectrum, and the more easily studied are the peculiar lines which Wollaston and Fraunhofer found crossing it. It was the presence of these black lines on the spectrum which led to the development of the spectroscope and established its significance and value. The work which the spectroscope does is simply to form an extended spectrum, but this spectrum varies with the different kinds of light admitted through the slit, the different kinds of light showing different arrangement of colored bands and dark lines, and such a definite relation between the light of various incandescing elementary bodies and their spectra has been found to exist, that the casting of a definite spectrum from the sun or stars indicates with certainty the presence in the sun or stars of the incandescing element which produces that spectrum. This application of the spectroscope is called _spectrum-analysis_, and by rendering any substance incandescent in the flame of a Bunsen burner, and directing the light of its incandescence through the spectroscope, its spectrum gives the basis of intelligent chemical identification. So delicate is its test that it has been