THE WAVE THEORY OF LIGHT. 103 



transmission quite as natural as the preceding' one, but which ofi'ers an 

 incomparable richness of resources. These are the waves of transverse 

 vibrations excited in an incompressible continuous medium; those 

 which explain all the properties of light. 



In this undulator}" mode the displacement of particles brings into 

 ])lay an elasticity of a special kind. This is the relative slipping of 

 strata concentric to th(^ disturbance wliich transmits the movement 

 and the oHort. The character of these waves is to impose on the 

 medium no variation of density as in the system of Descartes. The 

 richness of resource mentioned above depends upon the fact that the 

 form of the transverse vibration remains indeterminate, and thus con- 

 fers on waves an infinite variety of different properties. 



The rectilinear, circular, and elliptical forms characterize preciselj^ 

 the j)olarizations, so unexpected, which Fresnel discovered, and by the 

 aid of which he has so admirably explained the beautiful phenomena 

 of Arago produced by crystallized plates. 



The possible existence of waves which are propagated without change 

 of density, has profoundly modified the mathematical theory of elas- 

 ticity. Geometers found again in their equations waA'es having trans- 

 verse A'ibrations which were unknown to them. Thej^ learned besides, 

 from Fresnel, the most general constitution of elastic media, of which 

 they had not dreamed. 



It is in his admirable memoir on double refraction that this great 

 physicist set forth the idea that in crystals the elasticit}^ of the ether 

 ought to var}' with the direction, an unexpected condition and one of 

 extreme importance, which has transformed the fundamental bases of 

 molecular mechanics; the works of Cauchy and Green are the striking 

 proofs of it. From this principle Fresnel concluded the most general 

 form of the surface of the luminous wave in crystals, and found (as a 

 particular case) the sphere and ellipsoid that Hu}' gens had assigned to 

 the Iceland spar crystal. This new; discovery e-xcited universal admi- 

 ration among physicists and geometers. . When Arago came to expound 

 it before the Academie des Sciences, Laplace, who had been such a 

 long time hostile, declared himself convinced. Two years later Fres- 

 nel, unanimously elected a member of the Academy, was elected with 

 the same unanimity foreign member of the Royal Societ}^ of London. 

 Young himself transmitted to him the announcement of this distinction, 

 with personal testimony of his sincere admiration. 



The definite foundation of the undulatory theorj^ imposes the neces- 

 sit}^ of admitting the existence of an elastic medium to transmit the 

 luminous movement. But does not all transmission to a distance of 

 movement or of force imply the same condition ? To Faraday is due 

 the honor of having, like a true disciple of Descartes and Leibnitz, 

 proclaimed this principle, and of having resolutely attributed to reac- 



