of the Eye-pieces of Telescopes. 45 



Then, V = ^^"'^ ,, f, 



5 

 which, when n — 1,5, is -J". And 



2d. If the lens be plano-convex, in either position, v = ±f, 

 and V = Af; whence 



3F' 3F- 



Ex. 2. Suppose two lenses of equal focal length (M) are 

 placed in contact. 



Here C=M, B=-M, C'=— ; e=-~, e'=-— ; 

 ' 2 2 2 ' 



If n = l,5, the expression for /^ is 



7 /■ 5 \^ 25 

 Here then ^'= — ^ — (v+ - m) - -— m: 

 \2m\ 7 ^ 84 



12 mV 7 >' 84 



^ ' 12m V 7 7/21 



And 2 (i^ = - m. 

 Vn/ 3 



