of the Eye-pieces of Telescopes. 49 



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



first, F' = -— m. If equ 



in the opposite position^ F' = 



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



m 



4 



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

 convex, their plane sides towards the eye. This gives 



^ v//r^ 19 ^,.,. 25 



^^''^'i- ^(^)=,-^. ^m=i 



8 7V/- 



The next is, the first plano-convex, its plane side towards the eye, and 

 the second equi-convex, it gives 



^in--,m; Mf/)=3-^,; 2(Y) = 



36 M- 



6th. A single lens to produce the same effect must have tlie 



3 M 

 focal length — ; and, therefore, its smallest values of U and Y 



would be — vi, and 



9M' gyif- 



Ex. 4. Let the eye-piece be the common positive eye-piece, 

 ooii.si.sting of two lenses of focal length 3 M, placed at the distance 



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



lom V 7 / 1 



295 

 008 ' 



43 



7 ^ 1008 

 Vol.lU. Parti. G 



wi: 



