OPTICS is the science whose object is the investigation of the laws that regulate the phenomena of light and vision. The nature of light will be found treated of under LICHT, and its various properties under CHROMATICS, DIFFRACTION, INTERFERENCE, LENS, POLARIZATION, REFLECTION, REFRACTION, SPECTRUM, etc.; and we shall confine ourselves in this article to a historical sketch of the rise and progress of the science.
Optics, as a science, is entirely of modern growth, for though the Greeks and their disciples the Arabs had made some prbgress in mathematical optics, their knowledge was confined to the law of reflection and its more immediate consequences. Euclid, Aristotle, Archimedes, Hero, and Ptolemy were acquainted with the fact that light is 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 atmos pheric refraction. Alhacen (1070) and Vitellio the Pole (12601 were almost the only cul tivators of this science during the middle ages, and their additions to it were unimpor tant. The lens, though known from early antiquity, was not applied as an aid' to defective eye-sight till after the time of Roger Bacon. Jansen, Metius, and Galileo separately invented the telescope about the beginning of the 17th c.; and the last-men tioned philosopher, by its means, made various important astronomical discoveries. Kepler, a short time after, gave the true theory of the telescope, explained the method of finding the focal length of lenses, and applied it to find the power of the telescope, besides pointing out the mode of constructing ari 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 are inverted, a fact previously discovered by Maurolycus of Messina. From this period the science of optics steadily advanced, and its treasury of facts received numerous addi tions through the labors of De Dominis, Snell (the discoverer of the law of refraction in 1621), Descartes, Fermat, Barrow, Mariotte, and Boyle. Up to the time of Newton it was generally believed that color was produced by refraction, but that philosopher 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 tele scope underwent many valuable improvements, and in 1672 the description of his reflect~ in telescope was submitted to the royal society. Gregory had constructed an instru• meat on similar principles some years before. About the sauce time, Grimaldi made his interesting series of experiments on the effects of diffraction, and noticed the remarkable fact of the interference of one pencil of light with the action of another. Time complete
theory of the rainbow, with an elegant analysis of the colors of thin plates, and the hypothesis concerning the nature and propagation of light, now known as the "corpus cular" theory, completed Newton's contributions to the science. The important services of the ingenious but eccentric Hooke cannot be easily stated in such a brief extract, as lie 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, be forgotten that he has as much right as Huyghens to the credit of origin-at tog the undulatory theory, which is the favorite one at present. Time double refraction of Iceland spar was discovered (1660) by Bartholin, and fully explained in 1690 by Iluyghens, the propounder of the undulatory theory, who also aided the progress of mathematical optics to a considerable extent. The velocity of light was discovered by Romer (1675), and in 1720 the aberration of the fixed stars and its cause were made known by Bradley, who likewise determined with accuracy the amount of atmospheric refraction. Bonguer, Porterfield, Euler, and Lambert rendered essential service to phys ical optics; the same was done for time mathematical theory by Dollond (the inventor of the achromatic telescope), Clairaut, Dalenthert. Boscovich, etc.; while in later times the experiments of Delaval on the colors produced by reflection and refraction; the discus sion of the phenomena arising from unusual reflection or refraction, carried on by Vince, Wollaston, Biot, ldouge, and others; the discovery of polarization of light by _Mains (1808), and its investigation by Brewster, Biot, and Seebeck; of depolarization by Arcigo (1811), and of the optical properties as connected with the axes of crystals (1818) by Brewster; and the explanation of these and other optical phenomena, in accordance with the undulatory hypothesis by Young—the discoverer of the interference (q.v.) of rays— and Fresuel, went far to give optics a width of scope and symmetry which is possessed by few other sciences. The development of the undulatory theory and of optical 3cience generally has been carried on in the present century by Lloyd, Airy, Cauchy, and others; and more recently important discoveries in connection with the physical modifications and chemical properties of light have been made (the latter chiefly as far as the spec trum is concerned, by Kirchhoff), for a notice of which, and other discoveries, see PHOTOGRAPHY, SPECTRUM, and Other articles.