236 Mr. Airy on Achromatic Eye-pieces of Telescopes, 
F be the focal length, ; the power, of a lens: the expression for 
aa , G being a quantity 
glue: : g 1 
iF is required. It is known that P= 
independent of 7; 
If SF be required, since 
1 F 
tM ae a plirepug anes Pe = ( 1) + &e.}; 
FTF ea 
’ x on én \2 5 
dae a Spa n—1 J: 
or rejecting the square, &c. of — oF= If the 
product F should occur, F and f being the focal lengths of 
: : : Ie i 
two lenses of the same sort of glass, since 7 is — into 
70 “y =) , and ; into ke (a + =. the variation of me as far 
2 én 
5 Re: n—1- 
as the first power eee 
Similarly the variation of ep is eae &e. 
(3) On the achromatic object-glass. The principle of this 
is so well known, that we shall not step to describe it. Suppose 
then a convex lens of crown-glass, and a concave lens of flint- 
glass, to be placed in contact; let and 7m’ be the indices of 
refraction for mean rays, for crown and flint-glass respectively ; 
F and f the focal lengths of the lenses: the power of the com- 
, nae age . 
pound lens for mean rays is j — Pa for any other rays it is 
