Theories of Rotational Optical Activity. 751 



have in fact ample evidence that Drude's theory, even if 

 fundamentally sound, is nevertheless incomplete in detail, 

 because it omits the one term in the equation of motion of 

 the typical electron depending on the polarization in the 

 surrounding region whose presence has been definitely 

 established in connexion with the ordinary phenomena of 

 dispersion and absorption. 



I do not quite appreciate Dr. Bruhat's reference to Biot's 

 law, since this law still exists as an important factor in the 

 Lorentz form of the theory. The interest in this law lies in 

 the fact that it indicates in a general way the conditions 

 governing the phenomena and the general sort of explanation 

 necessary. It is, however, much too general to determine 

 the particular form of the theory which explains the further 

 and more special details found to be associated with the 

 phenomena. The main object of search now is to find some 

 form of theory which is consistent with the condition im- 

 posed by Biot's law, and which leads to formula? which 

 contain an explanation of the more particular regularities 

 observed in connexion with the phenomena; and Dr. Bruhat, 

 by his work, has probably helped us considerably nearer to 

 our ultimate object. 



In a footnote to the same letter Dr. Bruhat makes two re- 

 ferences to my paper on magnetic rotatory dispersion, quoting- 

 two facts, of which I am already aware and neither of which 

 is inconsistent with the formula} obtained in my paper. 

 The formula? I there obtain are based on an absolutely un- 

 assailable theoretical argument, and granted the truth of 

 certain plausible assumptions these formulae contain an 

 effective explanation of the results mentioned. The formula? 

 must therefore remain, although the assumptions on which 

 the deductions from them are based may possibly be proved 

 to be false. 



I must therefore still stand by the general views expressed 

 in my papers, although I am, of course, willing to acknow- 

 ledge definite proof of the inappropriateness of the particular 

 untried assumptions on which the theories under discussion 

 depend. But it is to be insisted that Drude's theory has 

 already, both directly and indirectly, been proved to be 

 incomplete. 



Yours truly, 



The University, Sheffield, Gr. H. LlVENS. 



October 12, 1014. 



