253 
determining the dispersive ratio of glass , &c. 
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 = 1066 = q, this gives 
_ 27 + 6 j + 7 
P — 6 (q + I) 9 
and the multiplier = 
t/ a 
/ 3 
da V a 3 
= 1 * 725 , 
= 2*218. 
Now, the aberration of our flint, which theoretically cor- 
rects the aberration of this plate, computed by this formula, 
viz. by taking q — 10, is 
/> = 
* 7 f+*l ±JL = 3-811 
6 (? + 1) 8 
whence = 
3-811 
- — = 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. 
