THE WAVE THEORY OF LIGHT. 101 



construction the path of the two rays, ordinary and extraordinary. 

 Experiment confirmed the results in every point. Laplace succeeded 

 in his turn (with the help of hypotheses of the constitution of lumi- 

 nous particles) to explain the path of these strange rays. The victory 

 of the theory of particles then appeared complete. A new phenome- 

 non arrived also appropriatel}- to render it striking. 



Malus discovered that a common ray of light reflected under a cer- 

 tain angle acquired unsymmetrical properties similar to those rays 

 from a crystal of Iceland spar. He explained this phenomenon by an 

 orientation of the luminous molecule, and, consequently, named this 

 light polarized light. This was a new success for emission. 



The triumph was not of long duration. In 1816 a ,young engineer 

 scarcely out of the Ecole Polytechnique, Augustin Fresnel, confided 

 to Arago his doubts on the theory then in favor, and pointed out to 

 him the experiments which tended to overthrow it. 



Supporting himself on the ideas of Huygens, he attacked the for- 

 midable question of rays and shadows, and had resolved it; all the 

 phenomena of diffraction were reduced to an analytical problem, 

 and observ^ations verified calculation marvelously. He had, without 

 knowing it, rediscovered Young's reasonings as well as the principle 

 of interference; Ijut more fortunate than he, he brought the experi- 

 iiientum, cruets — the two-mirror experiment; there, two rays, issuing 

 from the same source, free from an}" disturbance, produced when they 

 met, sometimes light, sometimes darkness. The illustrious Young 

 was the first to applaud the success of his young rival, and showed 

 him a kindness which never changed. 



Thus, thanks to the use of the two-mirror experiment, the theory of 

 Dr. Young — that is to say, the complete analogy of the luminoxis ray 

 and the sound wave- — is firmly established. 



Moreover, Fresnel's theory of diffraction shows the cause of their 

 dissimilarity; light is propagated in straight lines because the lumi- 

 nous waves are extremely small. On the contrary, sound is diffused 

 because the lengths of the sonorous waves are relatively very great. 



Thus vanished the terrible objection which had so much tormented 

 the mind of great Newton. 



But there remained still to explain another essential difference 

 ])etween the luminous wave and the sonorous wave; the latter under- 

 goes no polarization. Why is the luminous wave polarized? 



The answer to this question appeared so difficult that Young declared 

 he would renounce seeking it. Fresnel worked more than five years 

 to discover it. The answer is as simple as unexpected. The sound wave 

 can not be polarized because the vibrations are longitudinal; light, on 

 the other hand, can be polarized because the vibrations are transverse 

 that is to say, perpendicular to the luminous ray. 



Henceforth the nature of light is completely established. All the 



