of the Eye-pieces of Telescopes. 61 



thenr = 2lM, s= —M, r'=-^M, s'=---M: 



and 2(1/)=-^^, 2(Y)=-^i^. 

 ^ '' 1008 M' ^ ' 1008 M 



We shall not stop to find forms corresponding to the equation 



2(^) =0. 



, 2d. If the first lens be plano-convex, its plane side towards the 



23 1 70 



object-glass, V = m. If equi-convex, V = — — — m. If 



^ ^ 1008 ^ ' 1008 



1 SQ 



plano-convex in the other position, V = — — m. 

 ^ *^ ' 1008 



dd. If the second lens be plano-convex, its plane side toward 



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



21 



convex, the convexity toward the first, V = ■ m. 



•' ' 1008 



4th. The best combination of common lenses is, two plano- 

 convex lenses with the convexity of each turned towards the other : 

 they give 



504 ' ^ ' 504 M' ^ -* 504 M' 



The next is, either lens equi-convex, and the other plano-convex, 

 with its convexity towards the former: in this 



^(^)=-r^-^ ^(^)=T^' ^(^)-T^- 



5th. If an eye-piece of this construction, whatever be the 

 curvatures of the surfaces, be turned end for end, the values of 

 2(t/) and 2 (Y) will not be altered. 



G 2 



