424 HISTORY OF OPTICS. 



of this twist being different for different colours; a 

 result which was afterwards traced to a modifica- 

 tion of light different both from common and from 

 polarized light, and subsequently known as circular 

 polarization. Sir J. Herschel had the good fortune 

 and sagacity to discover that this peculiar kind 

 of polarization in quartz was connected with an 

 equally peculiar modification of crystallization, the 

 plagihedral faces which are seen, on some crystals, 

 obliquely disposed, and, as it were, following each 

 other round the crystal from left to right, or 

 from right to left. Sir J. Herschel found that the 

 right-handed or left-handed character of the cir- 

 cular polarization corresponded, in all cases, to 

 that of the crystal. 



In 1815, M. Biot, in his researches on the sub- 

 ject of circular polarization, was led to the unex- 

 pected and curious discovery, that this property, 

 which seemed to require for its very conception a 

 crystalline structure in the body, belonged never- 

 theless to several fluids, and in different directions 

 for different fluids. Oil of turpentine, and an essen- 

 tial oil of laurel, gave the plane of polarization a 

 rotation to the left hand; oil of citron, syrup of 

 sugar, and a solution of camphor, gave a rotation to 

 the right hand. Soon after, the like discovery was 

 made independently by Dr. Seebeck, of Berlin. 



It will easily be supposed that all these bril- 

 liant phenomena could not be observed, and the 

 laws of many of the phenomena discovered, without 



