THE FOCUS-APERTURE RATIO. 



293 



of N.A. 1*3 (optical index 10 '8) and the apochromatic 1/1 2th of 

 N.A. 14 (optical index 11-7), nor is there in catalogue values ; but 

 if a curve were drawn from actual object glasses, many fluorite 

 l/12ths would appear with optical index 9-0, for they often really 

 are l/14ths of N.A. 1-26. The curve would therefore be much 

 steeper than mine. In conclusion, I submit these three curves, 

 for achromats and semiapochromats, for apochromats, and for 

 160 



150 



140 



150 



120 



110 



100 



90 



80 



70 



60 



50 



40 



30 



20 



10 





-1 



•3 



•4 -5 -6 



Fig. 4, 



10 II 12 1-3 14 15 

 Fig. 5. 



oil-immersions, to the Club, as the first smooth curves that have 

 ever been drawn on a plan for the construction of microscope 

 objectives. 



When this Club was founded, teaching the use of the microscope 

 to beginners was one of its aims ; this paper, dealing as it does with 

 object glasses, invites such a course. Microscope objectives may 



