LIGHT. The word light is used in two distinct senses, namely, to designate the sensa tion which is characteristic of the organ of vision (q.v.), and also as a name for the usual cause of that sensation. This double meaning of the word would result in little inconvenience if there were always a definite relation between the sensation and its cause; but this is far from being true. For example, when we speak of white light, we may mean a sensation which is perfectly definite and familiar to all persons of normal vision, or we may mean that form of energy which can give rise to such a sensa tion. In this second sense the term is wholly indefinite, since there is an infinite variety of forms of energy which may give rise to the sensation of whiteness. The difficulty, which is a serious one in scientific language, may be avoided by restricting the use of the word to one of its significations, preferably to that of sensation, after the analogy of the use of the word sound. But such a restriction would not be in accordance with well-established usage, and it would necessitate the frequent employment of awkward circumlocutions. An other means of avoiding confusion is to so divide the subjects treated that the sense in which the word is used is unmistakable. This second method has the advantage of conciseness as well as that of being in accordance with the usage of most writers. The present article treats of light as a phenomenon of wave mo tion, wholly independent of the sense organ which betrays its existence to us. In it the eye is regarded as a simple optical instrument, quite like the photographic camera. This limitation admits of a satisfactory definition of the terms "white light," "yellow light," etc. Thus, by the former term we mean such waves as are emitted by a solid body at a very high temperature, as, for example, the incandescent lime in the lime-light. Any other kinds of waves, even if indistinguishable from these by the unassisted eye, are not white light. Again, yellow light, green light, etc., are the simplest waves which excite in a normal retina the sen sations yellow, green, etc.
Theories Concerning the Nature of Light. —It is now a little more than two centuries since the Dutch philosopher, Huyghens, pub lished a paper in which he explained the fa miliar phenomena of light by waves in a medium that pervades all space and is called the luminiferous ether. His reasoning was so
convincing, the explanations so simple, and the experiments supporting his views so apt, that except for the labors of the single philosopher then living who was greater than Huyghens himself, they could hardly have failed to re ceive at an early day the universal accept ance which they now command. Nine years earlier, in 1669, Newton had commenced his labors in the field of optics, by which, largely on account of fame and authority won in the domain of mechanics and astronomy, he estab lished a theory of light which remained al most unquestioned for nearly a century and a half. Newton supposed light to consist in ex tremely small particles of matter projected from shining bodies with enormous velocities. We now know that this hypothesis was not only less fruitful than that of Huyghens, but, even within the comparatively limited range of optical phenomena known to Newton and his contemporaries, was also less probable.
According to this view of Newton, visual sensations are produced by the impact upon the retina of minute corpuscles emitted from 1u .i nous bodies which pass freely through trans parent substances, differences of color, being due to differing size in these small bodies. When these corpuscles approach the boundary of an optically denser medium they are sub jected to a force of attraction which causes them to deviate from their otherwise rectilinear paths. This is the explanation of the phenom enon of refraction. The secondary phenome non of dispersion was very simple and naturally explained by an assumption that this attracting force varies with differing size. Singularly enough, the explanation• of one of the most common phenomena, that of partial reflection at the boundary of a transparent medium, offered formidable difficulties: How is an attraction which is necessary to account for refraction also to act as an apparent repulsion for those corpuscles which are thrown back from the surface in reflection? This is a difficulty which the advocates of the Newtonian theory have never been able to meet in a satisfactory manner.