X =U.T, 



._u 



7TI/JLZ 





26*0 Theory of an Imperfectly Homogeneous Elastic Solid. 



If i be the index of refraction, from air, of light having period 

 T, \ the wave-length in air, U the velocity of light in air, 



whence 



If L be the length of solid necessary to produce a complete 

 rotation of the plane of polarization, 



IT 



wherein we mav replace fi bv its value — . c 2 . 



A 



Sir W. Thomson has shown that the phenomenon of the 

 dispersion of light enables us to form an estimate of the inter- 

 molecular distances. But the phenomena of rotatory polari- 

 zation afford a vastly more delicate test of differences of wave- 

 velocity. In quartz, for example, whose rotatory effect is 

 about 24° per millimetre, the rotation of the plane of polariza- 

 tion reveals a difference of velocities amounting to less than 

 the 20,000th part of either. We may therefore hope to find 

 in these data some evidence regarding the distance ttt through- 

 out which the want of homogeneity of structure is sensible. 



The magnitude c 2 , used in the last equation, and introduced 

 in art. 9, is obviously of the form nt . &, where % is a quantity 

 of the same dimensions as c 2 v and may, for aught w^e know, be 

 of the same order of magnitude. The last equation may there- 

 fore be written 



m / c? \ 2X 1 



X 



\irk) 



But for the mean yellow rays of quartz, \) = 2oVo niillim., 



L = 15, i 





VSg/' 22500' (29) 



a result apparently in harmony with Sir W. Thomson's con- 

 clusions. 



