of the Eye-pieces of Telescopes. 15 



value of Q would have been ^ • J^- In the combination of two 

 lenses, the smallest value oi Q + Q' = R Jr R' is. 



3 ■ F' "^ 84 • F^ ~ 84 • F» ■ 



The distortion, then, is little more than one-third of what it would 

 be with a single eye-glass. 



Ex. 4. Suppose the eye-piece to be the Huyghenian eye-pie.-e ; 

 the first and second lenses having respectively the focal lengths 

 .3 M and M, and their distance being 2 M. 



1 .- _ ^ M 



2 



Let jj=m. Here C=F=3M; B'= -M, C'= —, 



By a process of the same kind Ave find 



I378 108 7 I j ' 



Hence, we find 



1st. R^R> or Q-(-Q' may be made = 0, by assuming any 

 value for v', and determining v. This eye-piece, then, may be made 

 absolutely free from distortion. 



2d. If the first lens be plano-convex, the convex side toward 

 tlie object-glass. 



