24 



Proceedings of the Royal Irish Academy, 



Comparing | = ^, 



e D' 



but from (10) — = -?r> 



where is a constant in the dispersion equation 



^2 = ^ + 



Substituting 



2ch" 



ha B, 



In practice h" and B' are expressed in the units corresponding to 

 wave-lengths 10"* cm. This does not change their ratio, and the 

 results can be exhibited in tabular form thus : — 



Substance. 



^1 



b" 



5i for 

 B line. 



Am for 

 B line. 



- X 10^ 



Carbon Bisulphide, . 



•0433 



•2379 



•762 



•042 



•53 



Creosote, 



•0277 



•380 



•745 



•024 



•77 



k = 3437-75, a = • 



