HISTORICAL SKETCH.
Geometrical optics is riot entirely a modern science, for the Greeks and their disciples the Arabs were acquainted with the law of reflection and its more immediate consequences. Aristotle, Archimedes, Hero, and Ptolemy knew that light was transmitted in straight lines; but with the important exception of Aristotle. and some of his followers, the ancient philosophers believed that rays proceeded from the eye to the object, instead of in the contrary direction. Ptolemy was well acquainted with atmospheric refraction. Alliazen (early part of eleventh century) and Vitellio the Pole (1260) were almost the only students of this science during the Middle Ages, and their additions to it were unimportant. The lens, though known from early antiquity, was not applied as an aid to defective eyesight till after the time of Roger Bacon. Jansen, Metins, and Galileo separately invented the telescope (q.v.) about the beginning of the seventeenth century; and with it the last-mentioned philosopher (see GALILEO) made various important astronomical discoveries. Kepler, shortly after, gave the true theory of the telescope, explained the method of finding the focal length of lenses, and applied it to find the magnifying power of the telescope, besides pointing out the mode of constrneting an instrument better adapted for astronomical purposes than that of Galileo; he also made some useful experiments on the nature of colors. and showed that images formed on the retina of the eye ac inverted, a fart previously discov ered by Maurolyens of Messina. From this period the science of optics steadily advanced through the labors of De Dominis, Snell (the dis •overer of the law of refraction in 1621), Des cartes. Fermat. Barrow, Ma•iotte. and Boyle. Up to the time of Newton it was generally be lieved that color was produced by refraction. but hue showed by a beautiful series of experiments that refraction only separates the colors already existing in white light. In his hands the theory and construction of the telescope many valuable improvements, and in 1672 the description of his reflecting telescope was sub witted to the Royal Society. Gregory had con structed an instrument on similar principles some years before. About the same time, Gri maldi made his interesting series of experiments on diffraction, and noticed the remarkable fact of the interference of one pencil of light with the action of another.
To explain the phenomenon of sharp shadows Newton advanced his 'corpuscular' theory of light, the idea of which was that light is due to the emission of streams of fine particles from the source of light. To explain refraction on
this theory it was necessary to assume that the velocity of light is less in air than in glass or water. Csing this theory, Newton tried to ex plain diffraction and the colors of thin plates; but the hypotheses involved in the explanation were too involved to he satisfactory. The im portant services of the ingenious hut eccentric Hooke cannot lie easily stated in a few words, as he discovered a little of everything,, completed nothing, and occupied himself to a large extent in combating faulty points in the theories of his contemporaries. It must not, however, he for gotten that lie has as much right as Huygens to the credit of originating the theory, though Hooke made little more than a lucky guess, while Huygens gave a remarkable discussion of the application of the theory to reflection and refraction. Newton's corpuscular theory was, however, the accepted one until the work of Young and rresnel. The double refrac tion of Iceland spar was discovered (1669) by Barth°lin, and fully explained in 1690 by Huygens. The velocity of light was discovered by Roemer (1675), and in 1720 the aberration of the fixed stars and its cause were made known by Bradley, who likewise determined with ac curacy the amount of atmospheric refraction. The fact that the two rays produced by Iceland spar were 'polarized' was known to Huygens; but polarization by reflection was not known until discovered by Mains in 1808. The proper ties of polarization were then investigated by Brewster, Biot. and especially by Arago.
The proof that light is due to wave-motion was first given by Dr. Thomas Young. who pub lished his work on interference in 1801. The difficulty of accounting for polarization phe nomena by the theory of waves was first met by Fresnel, who proposed the idea that the ether waves are transverse, and showed how this hypothesis perfectly explained the observations. It is to Fresnel also that we owe the explanation of rectilinear propagation and of diffraction. The attempt made by Fresno! to give a dynamical theory of light was not successful, although he did deduce formuhe for reflection. refraction, and tctal reflection which are in good accord with experiment. Green. Lord Kelvin, Helmholtz, Stokes, Rayleigh, and more recently Poincari•, Larmor, and Lorentz, have advanced dynamical theories with more or less success; the theory of Lorentz based on the motion of electrons is most satisfactory.