of the Eye-pieces of Telescopes. 47 



placed in the opposite position. In all these combinations, 



3d. If the first lens be plano-convex, its plane side towards 

 the object-glasSj 



v= -m, and V= m. 



' 84 



If equi-convex, V= O. If plano-convex in the other position, 



84 



4th. If the second lens be plano-convex, its plane side towards 



the first, F' = — — m. If equi-convex, V = —m. If plano- 

 84 84 



21 

 convex in the other position, F' — — ni. 



5th. The best combination of common lenses is, two plano- 

 convex lenses with their convexities together. This combination has 

 been considered. The next is, if either of the lenses be equi-convex, 

 and the other plano-convex, Avith its convexity towards it : this gives 



S(n=|lm=^, andE(C/)=-19 , 2(Y)=-^,. 

 ^ ^ 84 4' ^ ' 12M' ^ ' 12M 



6th. A single lens to produce the same effect, must have a focal 



M 



length — . Its smallest values of U and Y would be respectively 



14 , 34 



— j-i , and — j^ . 

 ?,M' 3M 



Ex. 3. Let the eye-piece be the Huyghenian eye-piece. 



Here C=3M, B'--M, C'= — , e= - ^, e'= ; 



' 2 6 ' 2 



