Internally Focussing Telescope in lacheometry. 897 

 and for /3 = 0, 



»{<°^>(B)' + HH)(K)} 

 =^(W)(b4 



It will be of interest to take the latter case in some detail. 

 When a negative focussing lens is employed, since by (17) 

 jy is necessarily positive, it is essential that 



<^>e-!)'^(W)(W)<°- 



Fig. 1. 

 20 



i-5 



i-0 



0-5 





i-0 



f-5 



20 

 B 



2-5 



30 



Internally-focussing Telescope with positive focussing lens recording 

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



This may be regarded as an equation giving a lower limit 

 to n or an upper limit to B, neither of which may be closely 



