due to the Scattering of Light by Electrons. 12") 



crystal. Incase (1) the values of l u l 2 , l z will vary from 

 molecule to molecule. If the orientation of the molecules 

 is quite irregular, the mean values of /,*, / 2 2 , / 3 2 are each 1/3, 

 while those of l^, /i/ 3 , f 2 h are zero: hence Q the mean value 

 of Q is given by 



Q^-JL [tex'z'Xev'-tex'y' . %ez'} 

 6A.C 



- -^-{Zex'y'tez'-tey'z' .Xex'} 



- ^{tey'z'Xex'—Xex'z' . Xet/}, 

 and the rotation for the solution is 



'27iQ a (8) 



1 obtained a similar expression, by a different method, for 

 this rotation in a paper published in the Proceedings of the 

 Cambridge Philosophical Society, xiv. p. 313 (1907). 



We see from the expression for Q that if it is to be finite 

 the molecule must be very unsym metrical. 



For Q vanishes 



(1) if the molecule has dynamical symmetry, for then 



A = B = C; 



(2) if the centre of the electrical charges coincides 



with the centre of mass, for then 



Zex ' = %ey ' = %ez ' — ; 



(3) if the principal axes of inertia coincide with axes 



of symmetry of the electrical charges, for then 



Se.v'y' = Xex'z' = Zey'z' = ; 



(4) if Zey , Zex'z=tez'2ea;'y , -2ex , %ey'z'. 



This relation would be fulfilled if the electrical 

 charges formed a geometrically symmetrical system, 

 even though the masses might not be symmetrical. 

 For such a system 



for all axes through the centre of figure ; hence 

 for parallel axes through the centre of mass 



Lex'z' =x z %e, 



