18 



Procceding^i of the Roijal IrisJi Acddony. 



K - A = 50r of the infra-red terms besides B^, 

 B, B, B^ 



A - ~ - -^--^-1= of the ultra-violet terms other than ^1 

 Aj Ao- A3- 



The values of (f), for the different kinds of ions whose periods 

 lie near the spectrum periods are almost identical. Prom a table 

 of dispersion constants the following averages were obtained. For 

 the ultra-violet periods c^i = 1. For the infra-red periods <^2 = 2'3. 

 The values of 20, and calculated from the table show that <j) 

 for the other ions present is small. The average value of 20, ^^"tl 20,. 

 is about "4. For the present approximation, it is evident that the 

 values of for any absorption-band do not greatly differ. We have 



1 



We can therefore write 



where ^ is of the same order of magnitude as the highest values 

 e\- 



of — occurring in v, since it has been shown above that in this 



approximation the coefficients are practically equal. Using the same 

 notation as before and equating the expression for 8 to the usual 

 expression with Verdet's constant expressed in minutes, we have 



