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 which 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. Mays reflected from water or from 
