150 
so much difference of colour for a given alteration of depth 
when the light is tinged with yellow as when it is white ; 
therefore the sensibility of the method is diminished. This 
may he put in another way. When the incident light is 
tinged yellow, the expression for the effective yellow after 
transmission through the cylinder is 
Y r - Yqt(rm 1 - px/i) 
Suppose the term in brackets to vanish, then the expression 
for the effective yellow becomes Y r, which is independent 
of the quantity of colouring matter and of the length of the 
column. 
I 
