DISCOVERY OF POLARIZATION. 155 



traversed the liquid undiminished. If the position of 

 the crystal was such, relatively to the plane in which 

 the reflexion took place, that the extraordinary ray was 

 partially reflected, then it was the ordinary ray which 

 was entirely transmitted. 



The phenomena of reflexion become thus a means of 

 distinguishing from each other rays polarized in opposite 

 directions. On the evening which followed the chance 

 observation of the sun's light, reflected from the windows 

 of the Luxembourg, Malus created one of the most 

 remarkable branches of modern optics, and acquired the 

 title which no one will ever contest to an immortal 

 renown. 



I should exceed the limits prescribed me, if I were 

 here to analyze all the observations which our colleague 

 made, in tracing the course of the direct and reflected rays 

 in which the phenomena of polarization were developed. 

 But I cannot omit, in order to prepare the reader for 

 understanding the curious facts with which Malus en- 

 riched the science in 1811, to give the definition of a 

 term which I shall have occasion to employ, that of a 

 ray " partially polarized." 



A ray of natural light always gives two images of the 

 same intensity, whatever may be the position of the face 

 of the crystal wliich it traverses, relatively to this ray. 

 A ray completely polarized, only gives one image in two 

 particular positions of the face of the crystal. A ray 

 partially polarized, possesses in some sort properties 

 intermediate to those of the natural and the completely 

 polarized ray. Like the natural ray, it gives always 

 two images ; and as with the polarized ray, these two 

 images have variable intensities, according to the posi- 

 tion of the analyzer. Rays i-eflected from water or from 



