AND A SIMPLE APERTOMETER DERIVED THEREFROM. 5 



It occurred to the author of this paper that Abbe's test might 

 be modified to project into the upper focal plane of an aplanatic 

 objective to be tested for numerical aperture — not for aplanatism — - 



Fig. 3. 



4* 



CHESHIRE'S APERTOMETER. 

 (a = 25 mm.) 



a series of equi-thick equi-distant concentric circles, each of which 

 should correspond to a definite and predetermined N.A. The 

 result is shown by Fig. 3. 



Theory of the New Apertometer. — Returning to Fig. 1, let us 



