1008 Rotational Optical Activity in Isotropic Media. 



further into the results here. It is, however, worth noticing 

 that in this case no change of sign of the rotation can take 



Ft O 



place on crossing the absorption band : this is the essential 

 difference between the two cases. The conclusion is, however, 

 precisely the opposite of that drawn from Drude's theory; in 

 fact on that theory the change of sign in the rotation takes 

 place only when an absorption band due to chirally active 

 electrons is crossed. 



The general formulae thus obtained, although rather com- 

 plicated, are nevertheless sufficiently general to contain an 

 effective explanation of all of the apparent anomalies in the 

 behaviour of the intrinsic optical rotatory power of isotropic 

 media. This theory, which makes the rotative power depend 

 essentially on the ordinary dispersion and absorption in the 

 medium, would indicate large variations of this rotative 

 power consequent on any variation of condition which alters 

 to any appreciable extent the usual dispersive and absorptive 

 properties of the medium. 



Of course it may be that the present theoiy is not general 

 enough to include all the details of a very complex pheno- 

 mena : this can only be tested by experiment. It appears, 

 however, to be of far more general applicability than an- 

 other theory of the phenomena yet proposed, and is therefore 

 deserving of a strict experimental examination to test its 

 completeness, before other assumptions are adopted to explain 

 what at first sight appear to be anomalies in the phenomena, 

 but which are all, or nearly all, probably involved in some 

 simple physical theory of the subject such as that developed 

 above. 



Sheffield, Feb. 4, 1914. 



Sote added April 22nd, 1914. — Since writing the above it 

 has been pointed out to me that the assumption B = H is not 

 valid in the present connexion. This appears to have been 

 noticed first by Voigt (Ann. der Physik, lxix. p. 307, 1899) 

 who finds that, in order that the equations of the theory may 

 conform to the energy principle in its usual form, it is 

 necessary to put 



ac at 



if terms in /S 2 and higher powers are neglected. 



It is therefore necessary to modify the form of the 



