Page:Popular Science Monthly Volume 36.djvu/47

Rh equally in all directions; and that the propagation of a wave front in any given direction is the result of a multitude of interferences among the elementary waves started from the particles which are successively disturbed. Accepting this principle, the laws of reflection and refraction, whether of light or sound, follow immediately; and they were worked out with great skill by Huygens. Another consequence is, that if an obstacle be interposed in the path of a wave, its edges must serve as new centers around which secondary waves will be propagated, while the main wave continues to advance. This is familiar in the case of water-waves.

If, therefore, light be due to wave-motion, no perfect geometric shadow is possible, for the shadow must suffer encroachment from these secondary waves thus diffracted. Such phenomena were actually observed in the case of light by Grimaldi, Hooke, and Huygens, but no satisfactory explanation was then given. It is surprising that Huygens did not think of applying the theory which had been so satisfactory in its application to other optical phenomena. He had not attempted to measure the length of waves of light, and had no conception of their exceeding minuteness. If any diffraction phenomena were to be observed, the encroachment for which he naturally looked was far greater than what had been noticed as inexplicable and almost imperceptibly narrow fringes. The absence of the diffraction phenomena such as he may have expected did not cause him to abandon his wave theory, though he could not but perceive that it constituted a stumbling-block. To the mind of Newton this obstacle was insuperable; it determined his rejection of Huygens's theory.

If Newton was not the inventor of the emission theory of light, he was certainly its most ardent advocate. It came into prominence along with the wave theory, or indeed a little after this; and by means of it very satisfactory explanations could be given of most optical phenomena. Newton's reasoning, and the authority of his great name, caused its acceptance by all contemporary physicists, except Hooke, Huygens, and Euler, and by all his successors for a century. Whichever of the two theories is accepted, assumptions are involved which are open to attack and incapable of being substantiated on any antecedent grounds. Its value has to be measured alone by its consistency with observed facts. It was not until about the beginning of the present century that Dr. Thomas Young revived the long-discarded wave theory, explained the diffraction of light by its aid, and showed the incompetency of the emission theory. His views were at first generally rejected, but in time they attracted the attention of Arago and Fresnel. The latter especially entered into the investigation with enthusiasm, and completed the establishment of the wave theory upon foundations that have never since been successfully assailed. The elastic