16 



Proceedings of the Boyal Irish Academy. 



ra = mass of the ion, 



\, = wave-length of ionic vibration, 



X = wave-length of light vibration. 



It was shown in the earlier part of this paper that, in many cases, 

 it was only necessary to take account of the ions in one ultra-violet 



absorption-band. For the rest of the ultra-violet terms 



0, and 



for Uhe infra-red terms In this case 



W2 = 1 + 2<^, + 



1 



x^ - Xr 



— < 



nil 



Hence we have 



J_rC;' B'X^ -j IVa!^ h"X^ 1 



and 



B^ 

 B 



(8) 



(9) 

 (10) 



Drude gives the following tables, which show how the formula (9) 

 agrees with observation : — 



V = 0-0450, 



Bisulphide of Carbon. 

 a" = + 0-1167, 



+ 0-2379.. 



Spectr. Line. 



S calc. 



Sobs. 



C 



0-592 



0-592 



I) 



0-760 



0-760 



E 



0-996 



1-000 



F 



1-225 



1-234 



G 



1-704 



1-704 



