determining the dispersive ratio of glass, &c. 267 
Whence the index 1 -f is known. 
As also, (p X {q1) = r* - - (3). 
=r . . (4). 
The radii sought. 
27. In order to determine the index of a given concave lens, 
we must combine it with any proper convex lens to produce 
a compound focus. Let this focus be (p, that of the convex 
lens/, and the required focus of the concave x, then by known 
principles 
X (p 
I 
T 
Whence x becomes known. Having then measured the radii 
of curvature as already stated, and calling them r, r', and 
index 1 -f- a, we have 
f~ + -V) = — , 
\ r ' r X ’ 
and since r, d, and x are known, a' and 1 4" ^ will of course 
be known also. 
