898 Accuracy of the Internally Focussing Telescope. 



approached. No such restriction occurs when the focussing: 

 lens is positive. Fig. 1 illustrates the results obtained when 

 the focussing lens is positive for various values of B with 

 0=1, 7 =-001, = 0, /=1000, n = 20; and fig. 2 the 

 corresponding results for a negative focussing lens with 

 7= —'001. All the calculations required are easily carried 



Fig. 2. 



20 



15 



1-0 



5 







1 





/ 





1 



1 



/ 





/ 









^l 













'JO 



1-2 



13 



1-4 



1-5 



Internally-focussing Telescope with negative focussing lens recording 

 distances between 20 and 1000 accurately to 1 part in 1000. 



out on a slide-rule. The systems illustrated in the diagrams 

 conflict with no mechanical restraints. The mean position 

 of the focussing lens approaches the stadia lines as B increases 

 in both series of telescopes. 



