222 FRESNEL. 



having once undergone this modification, the rays remain 

 single, or again subdivide into two, according to the direc- 

 tion in which they fall upon a second crystal presented to 

 them. But do these modifications show a relation exclu- 

 sively to double refraction ? do all their other properties 

 remain uninfluenced ? 



It was from the labours of one of our most distinguished 

 colleagues (like Fresnel, early snatched away from the 

 sciences of which he was the hope) that we have been 

 enabled to answer these important questions. Malus dis- 

 covered, in fact, that, in the act of reflexion, polarized 

 rays are differently affected from common rays : the lat- 

 ter, as every one knows, are partially reflected when they 

 fall even on transparent bodies, whatever may be the 

 angle of incidence, and whatever the position of the re- 

 flecting surface with respect to the sides of the ray. 

 When, on the contrary, the case is one of polarized light, 

 there is always one situation of the reflecting surface, 

 relatively to the poles, or sides, in which all reflexion 

 disappears if in this situation the reflexion take at a 

 particular incidence, which is different for each reflecting 

 surface, according to the nature of the substance of which 

 it is formed. 



If, after this curious observation, double refraction 

 ceased to be the only means of distinguishing polarized 

 from common light, at least it seemed to be the only way 

 by which rays of light could become polarized. But 

 soon a new experiment of Malus taught the scientific 

 world, to its great surprise, that there existed other 

 methods, far less abstruse, for producing this modifica- 

 tion. The most simple phenomenon of optics, the re- 

 flexion of light from a transparent mirror, is a powerful 

 means of producing polarization. Light, which is re- 



