248 Mr. Airy on Achromatic Eye-pieces of Telescopes, 
In one of Dollond’s 
p = 14,25 lines, g = 19, r= 22,75, s = 14, a= 22,48, 
b = 46,17, c = 21545. 
In one of Ramsden’s small telescopes, 
p= 0,77 inch, gq = 1,025, r= 1,01, s=0,79, a = 1,18, 
b=1,83, ec = 1,1. 
It appears from the formula, that the values of c in the several 
cases should have been 
2,31, 37,88, 19,37, 1,12. 
The course of a ray in the last of these is represented in 
Fig. 3.; the advantages which it possesses over the eye-piece 
with three Jenses are sufficiently obvious. 
(21). If it be required to find where the axes of the differently 
coloured pencils intersect each other: let z be the distance from 
the center of any lens, at which the axis of the pencil of mean 
rays is incident upon it = LM, Fig.4: ¢ the tangent of the angle 
made by that axis with the axis of the telescope = tan. MNL, and 
let x be the distance from the lens at which the differently co- 
loured axes cross: then it is plain from the figure that «dt=6:, 
or v= e. Thus, to find where the rays intersect after refraction 
at the second lens, we have, by (12), putting m for Dk, and sup- 
posing D very great, 
s=m(i-*): t=m er 
ma on 1’ 1 24a on ; ag 
—. : ‘ Ry = a ee 
Pena} Big nie PI at 28-2 = 
