SENSITIVE FLAMES AND SOUND-SHADOWS. 37 



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 imme- 

 diately ; 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 con- 

 tinues to advance. This is familiar in the case of water-waves. 



If, therefore, light be due to wave-motion, no perfect geo- 

 metric shadow is possible, for the shadow must suffer encroach- 

 ment from these secondary waves thus diffracted. Such phenom- 

 ena 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 impercept- 

 ibly 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 in- 

 superable ; 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 promi- 

 nence 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 author- 

 ity of his great name, caused its acceptance by all contemporary 

 physicists, except Hooke, Huygens, and Euler, and by all his suc- 

 cessors 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 founda- 

 tions that have never since been successfully assailed. The elastic 



