244 ROTATORY POLARIZATION. 



which comprises all the experimental laws of M. Biot (242). 

 The sign of the constant factor, (7, determines the direction of 

 the rotation. 



It is a striking peculiarity of this theory, that it contains 

 (in addition to the two refractive indices) but one constant, 

 and that this constant having been determined, from the 

 known angles of rotation when the ray traverses the axis of 

 the crystal, the ratio of the axes of the ellipses may be calcu- 

 lated when the ray is inclined by any angle to the axis. 

 The author has applied this calculation to the observations of 

 Sir Gr. B. Airy, and has found the calculated and observed 

 results to agree. 



(248) MM. Biot and Seebeck discovered that some of the 

 liquids, and even of the vapours, possess the same property as 

 quartz in the direction of its axis, and impress a rotation on 

 the plane of polarization of the intromitted ray, which is 

 proportional to the thickness of the substance traversed. The 

 fact is easily observed by transmitting a polarized ray through 

 a long tube filled with the liquid, and closed at each end by 

 parallel plates of glass, and analyzing the emergent ray by 

 a double-refracting prism. Among the liquids possessing 

 this property are oil of turpentine, oil of lemon, solution of 

 sugar in water, solution of camphor in alcohol, &c. The first- 

 mentioned of these liquids is right-handed, and the others 

 left-handed. They all possess the property in a much feebler 

 degree than quartz ; so that the ray must traverse a much 

 greater thickness of the substance, in order to have its plane 

 of polarization altered by the same amount. Thus a plate of 

 rock-crystal, whose thickness is one millimetre, rotates the 

 plane of polarization of the red ray through an arc of about 

 18 ; a plate of oil of turpentine, of the same thickness, 

 turns the plane of polarization only through a quarter of a 

 degree. 



