On the Limits of Resolving Power. By E. M. Nelson. 531 



the total aperture of the lens, which in most cases can never be 

 utilised. The table has been arranged according to ascending, 

 instead of descending values of N.A. Limits are given for each 

 0-025 N.A., from to 1 -0, after that for each 0-05 N.A. In the 

 table at the end of the Journal the low apertures advance by 

 0*05 N.A., and the wider apertures by 0*01; the result being, 

 that the percentage of the differences in the number of lines to an 

 inch, in the low apertures, is very great, and in the wide apertures 

 very small. At • 2 N.A. it is as much as 25 p.c., and at 1 *4 N.A. 

 it is • 7 p.c. The plan adopted in the present table tends to level 

 up these differences, for at 0'2 N.A. it is 11 p.c, and at 1*4 N.A. 

 it is 3£ p.c. 



A table somewhat similar to this has been of much service to 

 the author. This new one has been put forward in the hope that 

 it may be found useful by some of the Fellows of this Society. 



