POLARIZATION OF LIGHT. 153 



double-refracting prism towards the windows of the Luxem- 

 bourg palace, whioh then reflected the rays of the setting 

 sun. On turning round the prism, he was astonished to find 

 that the ordinary image of the window nearly disappeared 

 in two opposite positions of the prism ; while in two other 

 positions, at right angles to the former, the extraordinary 

 image nearly vanished. Struck with the analogy of this 

 phenomenon to that which is observed when light is trans- 

 mitted through two rhomboids of Iceland spar, Malus at 

 first ascribed it to some property which the light had ac- 

 quired in its passage through the atmosphere : but he soon 

 abandoned this idea, and found that this new property was 

 impressed upon the light by reflexion at the surface of the 

 glass. 



Pursuing the subject, he was led to the important disco- 

 very, that when a ray of light is reflected from the surface of 

 glass, or water, or any other transparent medium, at certain 

 angles, the reflected ray acquires all the characters which 

 belong to the light which has undergone double refraction. 

 "When received upon a rhomb of Iceland spar, or a double- 

 refracting prism, one of the two pencils into which it is di- 

 vided vanishes in two positions of the rhomb, namely, when 

 the principal section of the crystal is parallel or perpendicular 

 to the plane of reflexion ; while, in intermediate positions 

 these pencils vary in intensity- through every possible gra- 

 dation. 



A ray, then, may acquire sides or poles may, in short, 

 be polarized by reflexion at the surface of a transparent 

 medium, as well as by double refraction. The plane of po- 

 larization is the plane of reflexion at which the effect is pro- 

 duced ; and it is experimentally known by its relation to the 

 principal section of a double-refracting crystal, the ray 

 undergoing ordinary refraction only, when the principal sec- 

 tion is parallel to the plane of polarization. 



