408 HISTORY OF OPTICS. 



the principal plane of the second crystal were per- 

 pendicular to that of the first, and consequently 

 parallel to the sides of the refracted ray. This 

 view of the subject includes some of the leading 

 features of the case, but still leaves several consi- 

 derable difficulties. 



No material advance was made in the subject 

 till it was taken up by Malus 2 , along with the other 

 circumstances of double refraction, about a hundred 

 years afterwards. He verified what had been ob- 

 served by Huyghens and Newton, on the subject 

 of the variations which light thus exhibits ; but he 

 discovered that this modification, in virtue of which 

 light undergoes the ordinary, or the extraordinary, 

 refraction, according to the position of the plane 

 of the crystal, may be impressed upon it in many 

 other ways. One part of this discovery was made 

 accidentally 3 . In 1808, Malus happened to be ob- 

 serving the light of the setting sun, reflected from 

 the windows of the Luxembourg, through a rhom- 

 bohedron of Iceland spar; and he observed that 

 in turning round the crystal, the two images varied 

 in their intensity. Neither of the images com- 

 pletely vanished, because the light from the win- 

 dows was not properly modified, or, to use the term 

 which Malus soon adopted, was not completely 

 polarized. The complete polarization of light by 

 reflection from glass, or any other transparent sub- 



2 Malus, Th. de la Doub. Ref. p. 296. 



3 Arago, art. Polarization. Supp. Enc. Brit. 



