152 POLARIZATION OF LIGHT. 



surfaces. Such planes are denominated principal sections. 

 "When these sections are parallel, the ray which has under- 

 gone ordinary refraction by the first crystal will be also re- 

 fracted ordinarily by the second ; and the ray which has been 

 extraordinarily refracted by the first will be also extraordi- 

 narily refracted by the second. On the contrary, when the 

 principal sections of the two crystals are perpendicular, the 

 ray which has suffered ordinary refraction by the first crystal, 

 will undergo extraordinary refraction by the second ; and 

 the extraordinary ray of the first will be refracted according 

 to the ordinary law in the second. In the intermediate 

 positions of the two principal sections, each of the rays re- 

 fracted by the first crystal will be divided into two by the 

 second; and these two pencils are in general different in 

 intensity, their intensities being measured by the squares of 

 the cosines of the distances from the position of greatest in- 

 tensity. 



From this " wonderful phenomenon/' as Huygens justly 

 called it, it appears that each of the rays refracted by 

 the first rhomb has acquired properties which distinguish it 

 altogether from solar light. It has, in fact, acquired sides ; 

 and it is evident that the phenomena of refraction depend, 

 in some unknown manner, on the relation of these sides to 

 certain planes within the crystal. 



(165) The phenomenon discovered by Huygens remained, 

 for more than 100 years, a unique fact in science ; and the 

 kindred phenomena the properties which light acquires in 

 a greater or less degree in almost every modification which 

 it undergoes remained unnoticed until the beginning of the 

 present century. 



In the year 1808, while Malus was engaged in the 

 experimental researches by which he established the Huy- 

 genian law of double refraction, he happened to turn a 



