Six Lectures On Light

Principles of Spectrum Analysis -- Prismatic Analysis of the Light of Incandescent Vapors -- Discontinuous Spectra -- Spectrum Bands proved by Bunsen and Kirchhoff to be characteristic of the Vapor -- Discovery of Rubidium, Cæsium, and Thallium -- Relation of Emission to Absorption -- The Lines of Fraunhofer -- Their Explanation by Kirchhoff -- Solar Chemistry involved in this Explanation -- Foucault's Experiment -- Principles of Absorption -- Analogy of Sound and Light -- Experimental Demonstration of this Analogy -- Recent Applications of the Spectroscope -- Summary and Conclusion

     We have employed as our source of light in these lectures the ends of two rods of coke rendered incandescent by electricity. Coke is particularly suitable for this purpose, because it can bear intense heat without fusion or vaporization. It is also black, which helps the light; for, other circumstances being equal, as shown experimentally by Professor Balfour Stewart, the blacker the body the brighter will be its light when incandescent. Still, refractory as carbon is, if we closely examined our voltaic arc, or stream of light between the carbon-points, we should find there incandescent carbon-vapor. And if we could detach the light of this vapor from the more dazzling light of the solid points, we should find its spectrum not only less brilliant, but of a totally different character from the spectra that we have already seen. Instead of being an unbroken succession of colors from red to violet,