412 Proceedings of the Royal Society of Edinburgh. [Sess. 
and regard this as a function of X, it may be shown, according to the 
theories of Drude, H. A. Lorentz, and others, that the maximum change 
in n is one-half the maximum value of tik, which of course deter- 
mines the absorption. If (T + F — C)/T were ‘20 then, in the case of the 
measurements on fluorescein with the interferometer, uk should alter 
by 4*4 10~ 7 and n by 2*2 10 -7 , a wholly inappreciable amount on a 
length of 2 cms. 
In conclusion, a very simple construction of the molecule may be 
suggested to explain fluorescence in such a case as that of fluorescein, 
where we have a fluorescence band of somewhat longer wave-length 
than a well-marked absorption band. When a body is fluorescing its 
molecules are supposed to be in two states. In the first state let us 
