?24 Sir J. J. Thomson on some Optical Effects 



charge to the term in the expression for Y proportional to 



cos — (ci — as ) is proportional to 



A 



Bo 

 ' Oc 



(n 2 L — ajM) — cV r (n 2 %£o' — w^y')}, 



Thus Q the contribution o£ the whole molecule will be given 

 by the equation 



Q=-i c {( n!3 %'-»i 2 ^')(« 3 M- W2 N) 



"IS.! > 



"i< •» 



= — t- {wj«j— m 3 »2}(NS«y'- MS«5 ; ) 



If there are cr of these molecules per unit volume, the 

 rotation of the plane of polarization per uuit path is by 

 equation (1) equal to 



2ttQ- g (8) 



We must distinguish between the cases (1) when the 

 molecules are in the gaseous state or in solution, and (2) 

 when they form parts of a crystal. In case (2) we may 

 suppose that all the molecules are orientated in the same 

 way so that l u Z 2 , Z 3 are the same for all molecules ; then 

 equation (8) will giYe the rotation for light moving in the 

 direction determined by l lf / 2 , Z 3 , which are the direction 

 cosines of the path of the light relative to the axes of the 



