REFLEXION AND REFRACTION OF POLARIZED LIGHT 171 



Accordingly, the angle which the direction of the vibration 

 makes with the lines of the grating is less in the diffracted 

 than in the incident ray. 



It remains, therefore, only to measure the angles which the 

 planes of polarization of the incident and diffracted rays make 

 respectively ;with the lines of the grating. If the latter is less than 

 the former, the vibrations are parallel to the plane of polari- 

 zation ; if it be greater, they are perpendicular to it. The 

 experiment has been made by Professor Stokes himself ; and 

 he has drawn the conclusion that the latter is the fact, and, 

 consequently, that the hypothesis of Fresnel is the true one. 



The foregoing conclusion seems to be fully confirmed by 

 the phenomena of double refraction. In a uniaxal crystal, the 

 ordinary ray is polarized in the plane passing through the ray 

 and the axis of the crystal, and the extraordinary ray in the 

 perpendicular plane. But the velocity of the ordinary ray 

 is constant, and therefore also the elasticity of the medium 

 in the direction of the vibrations ; while, in the extraordi- 

 nary ray, both are variable with the direction of the ray. 

 Hence the direction of the vibrations, in the ordinary ray, 

 must be perpendicular to the plane passing through the ray 

 and the axis of the crystal ; while, in the extraordinary ray, 

 the direction of the vibrations is in the plane passing through 

 the ray and the axis. In both, therefore, the direction of 

 the vibrations is perpendicular to the plane of polariza- 

 tion. 



(183) We now proceed to consider the application of the 

 principle of transversal vibrations to the problem of reflexion 

 and refraction. 



The direction of the light reflected and refracted at the 

 surface of a uniform medium is a simple consequence of the 

 theory of waves ; and we have already explained Huygens* 

 demonstration of the laws which govern this direction a 



