REP'LEXION AND REFRACTION OF POLARIZED LIGHT. 169 



For, in this case, the force which resists the approach of two 

 strata of the fluid is much greater than that which opposes 

 their sliding on one another. 



(181) But the existence of transversal vibrations and of 

 transversal vibrations only is a necessary consequence of the 

 laws of interference of polarized light, if the theory of waves 

 be admitted at all. It has been experimentally proved, by 

 Fresnel and Arago, that two rays oppositely polarized com- 

 pound a single ray whose intensity is constant, whatever be 

 the phases of vibration in which they meet. But theory shows, 

 that the intensity of the light resulting from the union of two 

 rays oppositely polarized will be constant, and independent 

 of the phase, only ivhen the vibrations normal to the wave are 

 evanescent. 



This conclusion is easily extended to the case of common, 

 or unpolarized light. In unpolarized light, therefore, as in 

 polarized, the vibrations are only in the plane of the wave ; 

 but in the latter, these vibrations are all parallel to a fixed 

 line, while in the former they take place in every possible 

 direction in the plane of the wave. The phenomenon of po- 

 larization consists simply in the resolution of these vibrations 

 into two sets, in two rectangular directions, and the subse- 

 quent separation of the two systems of waves thus produced. 

 When the resolved vibrations are not separated, but one of 

 them is diminished in any ratio, the light is said to be partially 

 polarized. 



(182) We have stated that the vibrations of the molecules 

 of the ether, in a polarized ray, are all parallel to a fixed 

 direction in the plane of the wave. This fixed direction may 

 be either parallel or perpendicular to the plane of polarization; 

 and there is nothing in the phenomena, hitherto dis- 

 covered, to determine the choice between these two positions. 



