22 



Proceedings of the Royal Irish Academy. 



Note B. 



Calculation of and • 

 tJ± m 



The constants by which Drude calculates the magnetic rotation 

 are in arbitrary units. For magnetic rotation in general 8 = AJHdz, 

 where ^4^ is Verdet's constant in minutes. It is necessary to find the 

 factor to reduce Drude's constants to radians. Writing Si for his 

 rotation for any wave-length, and A^, Yerdet's constant, for the same 

 length, and k for the number of minutes in one radian, we get the 

 following relations : — 



Si = n + X^^TatJ ilypothesis of Rotating Ions. (4) 

 = - + Hypothesis of Hall-effect. (9) 



= A^a. 



ak 



A- (A- - A,2)_ 



nak [a^ (A2 - \,^y] ' 



This expression gives the rotation in radians for any magnetic 

 field and any thickness of the substance in terms of the constants 

 given by Drude, and can be directly compared with either of the 

 general theoretical formulae.. 



Calculation of 



The formula for the rotation deduced from the hypothesis of 

 rotating ions can be put in the following form (4) in the case of 

 certain substances 



and the experimental constants give a general formula in the form 



