LIGHT 435 LIGHT, that force in nature which, acting j on the retina, produces the sensation of vision. It also has an important influence upon chemi- cal affinity, as may be instanced in the union of hydrogen and chlorine gases, which in dif- fused light takes place gradually, but in the direct rays of the sun instantly. The manifes- tation of vitality in plants is almost entirely dependent upon it, and most animals cannot maintain their health for any considerable time without its presence. The sources of light are self-luminous bodies, such as the sun, the fixed stars, certain meteors, those planets which have not cooled below the point of redness, and ter- restrial bodies in a state of incandescence and phosphorescence. The ancient Greeks were aware that rays of light proceeded from illu- minated objects in straight lines, which were reflected as well as refracted by surfaces ac- cording to certain definite laws. But all the ancient philosophers had very inconsistent no- tions in regard to its connection with vision, believing this vital function to be performed by something which proceeded from the eye to the object; and it is remarkable that this illogical idea was entertained until the early part of the llth century, when it was refuted by the Arabian astronomer Alhazen, who seems to have been the first to perceive that vision is produced by rays of light proceeding from the object to the eye. Two principal theories have been advocated to account for the phe- nomena of light, the emission or corpuscular theory, and the undulatory theory. The emis- sion theory, which was the first to be con- nected with optics on mechanical principles, originated with Descartes, who was the founder of modern mechanical philosophy. He con- ceived light to consist of small particles emitted by luminous bodies, capable like elastic balls of bounding from or being reflected by surfaces ; and he explains the production of colors by as- suming that a rotary motion is given to these particles under certain circumstances. But Newton was the founder of the emission theo- ry, because he developed nearly all the doc- trines by which it was maintained for more than a century, and also discovered many of the laws of optics by its means. The principal distin- guishing hypotheses of this theory will be no- ticed in the course of this article. The undu- latory theory assumes that the space between the celestial bodies is occupied by a kind of imponderable matter, which is infinitely elastic and of extreme tenuity, so that it not only occupies the space between bodies, but also enters into them and performs its function of undulation within them and between their par- ticles. This subtile matter is called the lumi- niferous or cosmic ether (see ETHEE), and the luminousness of a body is assumed to be due to a rapid vibratory motion of its molecules which is propagated in the ether in the form of waves. These waves proceed in all direc- tions from every luminous point, resembling in that respect the waves of sound ; the luminous . FIG. 1. point, like that of the origin of sound, being the centre of a sphere. The waves, however, are propagated in different ways in the two cases. The sphere of sound is formed by alter- nate expansions and condensations of the air, the waves consisting of concentric shells of alternate density and rarity, and the mo- tion of the aerial par- ticles being to and fro in the direction of the radii ' of the sphere. In the case of light the propa- gation is also in the direction of the radii, but the motion of the particles of ether is supposed to be in a transverse direc- tion, as represented in section in fig. 1. The transverse oscillations in a line of ether par- ticles proceeding in a right line from the source of light, as from c to a, is called a ray, and that length of a ray which at any instant includes all the phases of an oscillation is called a wave ; while the form of that part of the wave which is presented toward the direction of propagation is called a wave front. In the figure the vibrations are represented as taking place in one plane. Some regard each ray as cylindrical in form, and made up of a number of transverse vibrations which cross each other like the diameters of a cylinder ; but it is only necessary to suppose that each ray vibrates in one plane, and that there are innumerable parallel rays with planes at every inclination to each other, as well as rays crossing each other in all directions. The velocity of light is known to be about a million times as great as that of sound, so that upon the undulatory theory air, or any other ponderable form of matter with which we are acquainted, would not be a sufficiently subtile medium for its prop- agation. For the purpose of explanation it there- fore becomes necessary to assume some other medium which possesses adequate mechanical properties. Such a medium had been imagined by the ancients, and Hooke in 1664 proposed a theory that "light is propagated by a quick, short, and vibratory motion, in a homogeneous medium, in such a way that every pulse or vibration of the luminous body will generate a sphere which will continually increase and grow bigger, as the waves or rings on the sur- face of water do swell into bigger and bigger circles about a point in it." His theory, how- ever, contained many erroneous hypotheses, and was unsupported by experimental or mathematical proof ; but in the hands of Huygens it soon assumed a form capable of explaining most of the phenomena of light in accordance with established mathematical prin- ciples, and of standing the test of experimental demonstration to the present time. No better idea of the inception of the undulatory theory
