SPECTROSCOPY. If one observes a Bunsen flame through an ordinary glass prism held close to the eye he will see a succession of colored images of the flame. This series of colored images is called the spectrum of the flame. An instrument, such as the prism, which will separate the various radiations emitted by any source of light, is called a spectroscope. Spectroscopy is, therefore, defined as that science which has for its object the determina tion and description of the various radiations which different bodies emit, reflect and absorb. The subject is here treated under seven head ings as follows: (1) Preliminary, concerning the Nature of Light; (2) Sources of Radiation; (3) Separation of Radiations; (4) Methods of Recording Radiations; (5) Comparison and In terpretation of Radiations; (6) Summary of Principles; (7) Bibliography.
1. Nature of Light.— Present views con cerning that form of energy which we call light are based almost wholly upon experimental evi dence furnished by Huygens, Newton, Young and Fresnel. Since light is the subject matter of spectroscopy, it is essential, at the outset, that we should understand the modern theory of light as perfected by these four men. Huygens, by assuming that light consists of a wave-mo tion in the ether, showed that on simple dynami cal principles, reflection, refraction and the finite velocity of light might be easily explained. He first interpreted for us the physical constant known as the "refractive index," suggesting that for 'any medium it is merely the ratio of the speed of light in air to the speed of light in that medium. To Newton we owe two capital dis coveries concerning the nature of light. In 1669 he demonstrated experimentally that white light contains all the colors of the rainbow, that the simple prismatic colors obtained by analyz ing white light cannot be farther separated into other colors by passing them through prisms, and that color, in general, is merely white light from which has been removed some of its colored constituents. Besides this, Newton
established the fact that these luminous dis turbances (which he thought of as moods of easy and of difficult transmission, but which we now call light-waves), are not irregular pulses, as Huygens imagined, but are periodic in char acter. This inference is drawn from the phe nomenon known as "Newton's Rings." We come next to the work of Thomas Young (1773 1829) who first demonstrated the fact that, under proper conditions, two rays of light, that is two trains of light-waves, may interfere in such a manner as to produce darkness. Young's explanation of diffraction, however, proved to be utterly insufficient, and for the phenomenon of polarization he could offer no explanation whatever, for the reason that he conceived light-waves to be longitudinal and not transverse vibrations. To summarize Young's work most 'briefly would be to say that he introduced into optics the principle of in terference— or of "non-interference," as sug gested by Michelson, since the principle con sists merely in the fact that of two light rays meeting at a point each produces its own effect independently of the other. Fresnel (1788 1827), in a certain sense, perfected the wave theory of light by combining the principle of Huygens with the principle of interference and by introducing the idea that light-waves, unlike sound-waves, are transverse disturbances. Evi dence for this view lay in the fact that light can be polarized. We may now summarize the labors of these four men by saying that experi ment seems to indicate that light consists in a transverse wave-motion in a medium which per vades all known space and which we call the ether. In space devoid of ordinary matter light of any color travels with a speed of approx imately 186,330 miles per second, while in all ordinary matter light travels with less speed, the amount of diminution becoming greater and greater as we pass from the red to the violet.