938 Mr. Airy on Achromatic Eye-pieces of Telescopes, 
gaa. , the compound Jens, as before, will be achro- 
matic, upon making 
F Bot _ ¢e(F- a) 
Gas Ngee oe 
(5) The following remark is worthy of notice. In an achro- 
matic object-glass we have found, when the lenses are in 
contact, f=cF; when the lenses are separated by the space a, 
f= aos a) 
if, eis the lenses are in contact, the focal length of the concave 
lens be too short, or its refraction too great, or (in the language 
of opticians) the colour be over-corrected, the object-glass may 
be made achromatic by separating the lenses. This important 
theorem is well known to every scientific workman. If the focal 
length of the concave Jens be too great, no alteration will make 
the object-glass achromatic. 
(6) Upon comparing the dispersions of different media, it 
a quantity evidently less than the former. Hence 
én : 
appears that ——— may be represented with an accuracy great 
on \? 
Se 
i+ c= 
Suppose an object-glass to be formed of three lenses of different 
substances: let n, n’, n”, be their refractive indices for mean rays, 
and let 
é e apnoe 2.n0 bao av Alty ‘ 
n' +2(2* : n n a. n ) : 
—— =C ._— 
> eat a a n-1 n—1 
enough for all practical purposes, by the form c. at 
then if f, f’, 5 be the focal lengths for mean rays, the power of 
the object-glass for rays of any other colour will be 
us ra) +7 Ore a) tpt =) 
eens. ptp) mart Gtp) Ga 
a 
