LIGHT

is that branch of science whi deals with the external cause of ¢ visual sensations. It is also known Optics, and is frequently divided ir such © sections as Geometrical Opti Physical Optics, and (although beyo the present scope) Physiological Optics. The first of these deals with the transmi ion, reflection, and refraction of light rays, without inquiring into their 4 physical nature. The second explains all phenomena relating to light on the theory that it is due to a periodic dis_ placement or wave motion in a lum in_ iferous medium. The third includes the the study ‘physiological of anatomy of factors the human relating eye and to human vision. Previous to the beginning of the 19th cent., several theories had been propounded to account for the nature of light and its propagation, but the two _ principal explanations offered were those - known as the Corpuscular Theory and the Undulaiory or Wave Theory. According to the former, the sensation of light was excited by the impact on the retina of a large number of minute particles, or corpuscles, emitted in Jarge numbers by the source of light and traveling through transparent substances, as well as vacuous space, with great speed. Apart from inherent improbabilities, there were fatal objections to this theory—(e.g.) the speed of light should, according to it, be greater in a denser medium, whereas Foucault showed experimentally that the reverse fs the case. The theory encounters serious difficulties in attempting to explain interference, diffraction (2.e., the bending of light rays round the edge of a narrow slit and the formation of alternate light and dark bands bordering the image on a screen), and polarization. The undulatory theory supposes that light consists of wave motion in a medium filling all space, including intramolecular space, and that the vibrations are perpendicular to the direction of the ray. At first this theory required the a umption of a medium po e ing density and elasticity; but the almost universal belief now is that light is an electric phenomenon, the vibrations being electric and magnetic displacements. This theory, which we owe to the genius of Maxwell, led to a new conception of the meaning of time; and this, again, has recently, in the hands of Winstein, played an important part in the development of the theory of Relativity. One striking result of Einstein’s doctrine is that a ray of light otherwise straight suffers deflection in the neighborhood of a large gravitating body like the sun. On May 29, 1919 astronomers pe estapbed the heavens in the neigh or hood of the sun during a total eclipse and obtained measurements of star positions which verified Einstein’s prediction. Further confirmation was given in 1923 from the results of photographing the total eclipse of the sun in 1922. Accepting the undulatory theory, we may briefly summarize the leading points in the science by considering the e ential characteristics of a ray of light, and, to begin with, we confine ourselves to monochromatic light. Any wave which is propagated in a medium has three principal features (see Wavn). It must have (1) a certain wave-length, just as regular waves in the ocean have a length measured from crest to crest; (2) @ certain pertod—(t.e.), the time taken by any portion of the medium affected by the wave to describe one complete vibration; (3) a certain amplitude—(i.e.); the displacement, elastic or electric (according to the view taken), as mease ured from the position of equilibrium. The wave-length of light which is ordinarily appreciable by the human eye varies from 37,5 to z millimetre, The former (7.e., the shorter) waves belong to violet-colored light. As the wave-length increases we have, by ine definitely minute gradations, blue, green; and orange light, until for the longer wave-length mentioned we have red light. This range does not, however! exhaust all po ible wave-lengths, for the existence of waves which are either too short or too long to affect the retina has been fully proved. The amplitude of a light wave is the factor which governs the intensity of the ray, for with light of a given wavee length the energy in the ray is prop or tional to the square of the amplitude.' The speed of light in space is 186,330 m. per second, and is the same for all wave-lengths, transparent matter, the velocity of light is reduced and varies with the wave-length. Tho simplest properties of undulatory motion explain important phenomena which arise when rays from two separate sources meet at a point. Anal a go us cases are found in ocean waves, in sound, and in the phenomena of the tides. If, owing to any cause, two series of waves from different sources meet on the same water surface, there may occur the case where crests of waves in one series unite with crests of the other series. The resultant wave has then an amplitude equal to the sum of the component amplitudes. But where the crests of one series meet the troughs of the other, the resultant amplitude is the difference of the component amplitudes, and if these are equal there is no disturbance of the sea level. Similarly, it is po ible to produce a combination of rays which will give either increased or. diminished brightne . This is known as interference. Another group of phenomena is due to the po ibility of the vibrations in the ether being confined to one particular plane. The light is then said to be plane polarized. Plane eer light is best obtained from Iceland spar, which has the property of double refraction— that is, of dividing any transmitted ray of ordinary light into two rays, each of which is polarized. The explanation of double refraction is one of the triumphs of the wave theory. Further, since harmonic motions in different directions combine to produce circular or elliptic motion, we can combine polarized rays in a similar manner. As long as a ray travels in the same homoeneous medium, it does so in a straight e; but when it arrives at the surface -Of separation between two media whic h| ‘are optically different, a change takes place. One portion of the ray may be thrown backwards into the medium in be absorbed by the second medium and its energy transformed into heat. At present the line of advance in physical optics is towards a satisfactory explanation as to how luminous matter transfers energy to the ether so as to produce vibratory motion. The most promising explanation is that a ociated with each atom (or perhaps constituting each atom) there are electrically charged particles or electrons, whose ma is mostly, if not wholiy, electromagnetic, and whose motions give rise to ether waves. What is known as aberration of light in optical instruments means the devia- ‘tion of part of a pencil of rays from the theoretical focus. It is of two kinds, spherical and chromatic. The subject is further treated under Lays. ‘LIGHT CURE, use of ultra-violet \Pays in such diseases as lupus, favus, etc. RY LIGHT FOOT, JOHN (1602-75), Eng. theologian; ed. at Cambridge; member of Westminster A embly, 1642-44: best known as Hebrew and Rabbinical scholar; his greatest work was Horae