UNDULATORY THEORY OF LIGHT, a theory in which it is attempted to explain the phenomena of light by the supposed vibra tions of an ethereal medium.
Descartes is considered as the first who entertained the opinion that vision might be so explained ; but that philosopher only states that light may be a certain movement or action of the molecules of air and other pellucid substances. He supposes that the effects may be instantaneously transmitted to the eye; and he compares the appre hension of external objects by vision to that which a blind man obtains when, holding a staff at one of its extremities in the hand, the opposite extremity conies in contact with an obstacle. (‘ Dioptrices; cap. 1.) 3Iallebranche appears to have conceived th it there existed an analogy between the phenomena of sound and those of light ; ascribing the former to vibratory movements of the particles of air, and the latter to the like movements of the particles of an ethereal medium between the luminous body and the eye. But Huygens (` Tmctatus de Lumine ') both advanced the undulatory hypothesis and explained by it the laws of reflection and refraction of light, not only for ordinary media, but also for Iceland spar, which possesses a powerful double refraction. Newton, however, adopted the corpuscular theory, having been influenced it would seem chiefly by the difficulty of accounting for the rectilinear propagation of a pencil of light, and the existence of shadows, on the undulatory hypothesis. The great reputation of Newton, the simplicity with which the existence of rays fell in with the ourpuocular compared with the undulatory theory, the want of sufficient 1. 'Rarity with the conception of undulations, and the mathematical difficulties inherent in their investigation, conspired to retard the progress of the undulatory theory, and caused the cor puscular theory to be that chiefly in vogue up to the commencement of the present century. About that time the undulatory theory was revived by Dr. Young, who, guided by the analogy of sound, was led to the discovery of the important principle of interference (Item. entesce], and applied it successfully to the explanation of the colours of thin plates, and of the fringes seen in the middle of the shadow of a slender opaque body, which latter wore proved to be incontestably duo to interference, since they disappeared when the light which passed, or was about to pass by one side of the opaque body was intercepted by a screen, though the light which passed by the other side remained the same as before. The same principle, in the hands of Dr. Young, led to an explanation far more complete than any that had hitherto been given of various others of the curious phenomena of diffraction (Drrrueerearr OF Lima], and at his suggestion Dr. Wollaston undertook an experi
mental investigation of the laws of extraordinary refraction in Iceland spar, which ended in a complete verification of the construction which had given under the guidance of the theory of undulations, a construction which was further verified by Marius in France, by observations rude in a totally different manner, Later still. Fresnel, in his celebrated memoir on diffraction, reduced the explanation of the phenomena of diffraction to two principles, 11 uyghens's principle, and the principle of interference, which are neces sary consequences of the most fundamental assumptions of the undu latory theory, and proved by exact meaeures the accordance of theory and observation ; and Fraunhofer, by observations on pure diffraction spectra, admitting of almost astronomical precision, verified the formula which result for that case from the principle of interference. Mean while a new and splendid class of phenomena, those relating to polarisation and the colours of crystalline plates, &c., engaged the attention of the most celebrated experimentalists ; and though these for a time seemed difficult of explanation on any theory, they fell naturally into their places when the hypothesis of transverse vibrations [POLARISATION or Ltour), which occurred independently to Young and Freanel. was introduced into the undulatory theory. Guided by this hypothesis, and by dynamical considerations, Fresnel oonstructed his celebrated theory of double refraction, which, without being, or professing to be, a perfectly rigorous mechanical theory, is one of the moat wonderful scientific generalisations which the human mind has achieved ; and while it explained the previously known laws of double refraction. and the polarisation of each of the refracted rays, corrected in some respects the laws previously assumed by experimeutalista, and led to the discovery of new and unexpected phenomena. Thus while in the progress of optical science the corpuscular theory remained almost entirely barren of results, beyond the explanation of the laws of reflection and refraction and of the aberration of light, and even appeerel to be contradicted by certain phenomena, the theory of undulations has continually been acquiring fresh strength, by bringing complicated phenomena into harmony with one another, and with the fundamental 113 potheries of the theory. And within the last few years M. Frttetult, by !moving, by direct experiment, that light travels faster in air than in water, has given the coup de grace to the corpuscular theory.