determining the dispersive ratio of glass^ ^c. 253 
Messrs. W. and T. Gilbert I have been enabled to make 
various experiments, some of the most useful of which I 
will endeavour to describe. 
First, however, let us ascertain what multiplier we should 
require, according to the practice we are speaking of, in the 
particular example computed in a preceding page. 
16. Assuming our plate lens such as we have found it, viz. 
having-^ = i-o 66 = q, this gives 
_ 27 9®+ 6^+7 
P — 6(q + ir 
and the multiplier = 
= i'72S> 
» 
= 2*218. 
Now, the aberration of our flint, which theoretically cor¬ 
rects the aberration of this plate, computed by this formula, 
viz. by taking ^ = 10, is 
P • 
3-811 
+ =3-811 
whence 
p 
1-725 
6 (9 + 1)* 
= 2*209. 
The empirical rule therefore approaches extremely near 
in its result, in this case, to that obtained on strict optical 
principles ; and in several other comparisons I have made, 
the agreement has been found equally close, although in 
others it differs too widely to be depended upon ; and as the 
rule which I have given is strictly correct, and involves no 
greater difficulty of calculation than that we have been ex¬ 
amining, there can, I think, be no doubt to which the pre¬ 
ference should be given in any practical case of this kind. 
