462 HISTORY OF OPTICS. 



double refraction, and Young, knowing the resources 

 of his own theory, were the first persons to enter 

 upon this undertaking. M. Biot's theory, though in 

 the end displaced by its rival, is well worth notice 

 in the history of the subject. It was what he called 

 the doctrine of moveable polarization. He con- 

 ceived that when the molecules of light pass through 

 thin crystalline plates, the plane of polarization 

 undergoes an oscillation which carries it backwards 

 and forwards through a certain angle, namely, twice 

 the angle contained between the original plane of 

 polarization and the principal section of the crystal. 

 The intervals which this oscillation occupies are 

 lengths of the path of the ray, very minute, and 

 different for different colours, like Newton's fits of 

 easy transmission; on which model, indeed, the new 

 theory was evidently framed 16 . The colours pro- 

 duced in the phenomena of dipolarization really do 

 depend, in a periodical manner, on the length of the 

 path of the light through the crystal, and a theory 

 such as Biot's was capable of being modified, and 

 was modified, so as to include the leading features 

 of the facts as then known ; but many of its condi- 

 tions being founded on special circumstances in the 

 experiments, and not on the real conditions of 

 nature, there were in it several incongruities, as 



10 See MM. Arago and Biot's Memoirs, Mem. List, for 1811 ; 

 the whole volume for 1812 is a Memoir of M. Biot's (published 

 1814); also Mem. Inst. for 1817; M. Biot's Mem. read in 1818, 

 published in 1819, and for 1818. 



