174 The Aether as an Elastic Solid. 



phenomenon was studied shortly afterwards by Biot,* who 

 showed that the alteration consists in a rotation of the plane of 

 polarization about the direction of propagation : the angle of 

 rotation is proportional to the thickness of the plate and 

 inversely proportional to the square of the wave-length. 



In some specimens of quartz the rotation is from left to 

 right, in others from right to left. This distinction was shown 

 by Sir John Herschelf (b. 1792, d. 1871) in 1820 to be 

 associated with differences in the crystalline form of the 

 specimens, the two types bearing the same relation to each 

 other as a right-handed and left-handed helix respectively. 

 FresnelJ and W. Thomsong proposed the term helical to 

 denote the property of rotating the plane of polarization, 

 exhibited by such bodies as quartz : the less appropriate term 

 natural rotatory polarization is, however, generally used.|| 



Biot showed that many liquid organic bodies, e.g. turpentine 

 and sugar solutions, possess the natural rotatory property : we 

 might be led to infer the presence of a helical structure in 

 the molecules of such substances ; and this inference is sup- 

 ported by the study of their chemical constitution; for they 

 are invariably of the "mirror-image" or "enantiomorphous" 

 type, in which one of the atoms (generally carbon) is asym- 

 metrically linked to other atoms. 



The next advance in the subject was due to Fresnel,1[ who 

 showed that in naturally active bodies the velocity of propa- 

 gation of circularly polarized light is different according as the 

 polarization is right-handed or left-handed. From this 

 property the rotation of the plane of polarization of a plane- 

 polarized ray may be immediately deduced ; for the plane- 

 polarized ray may be resolved into two rays circularly polarized 

 in opposite senses, and these advance in phase by different 



* Mem. de 1'Institut, 1812, Part i, p. 218, sqq. ; Annales de Chim., ix (1818), 

 p. 372; x (1819), p. 63. tCamb. Phil. Soc. Trans, i, p. 43. 



J Mem. de 1'Inst. vii, p. 73. Baltimore Lectures (ed. 1904), p. 31. 



|| The term rotatory may be applied with propriety to the property discovered 

 by Faraday, which will be discussed later. 



H Annales de Chim. xxviii (1825), p. 147. 



