76 On the Refractive Powers, &c. 
nicety with which the focal distances can be 
ascertained. A hollow glass prism to contain 
the acid might perhaps be substituted in place 
of the lenses, with advantage; though some 
uncertainty might arise from the variable re- 
frangibility of the rays of light passing through 
acids of different densities, and which might 
create some difficulty in comparing the dif- 
ferent situations of the spectrum ; this objec- 
tion applies of course in a certain degree in 
the use of lenses, but by making use of such 
as have aconsiderable focal length, it becomes 
so trifling as not to be of material consequence 
for this purpose. 
