THE PRESIDENT'S ADDRESS. 29 



We have therefore three limits, viz. : — 

 *Square aperture (same as Abbe) .. .„ $= 



Circular „ (Rayleigh experimental) 8 

 Circular „ (Airy calculated) ... 8 



A 



109 AF 



A 

 1-2197 AF 



A 



It is stated that there are certain theoretical considerations 

 which show that a star disc as seen in a telescope should be 

 smaller than that calculated by Airy ; this reference, however, 

 I have not been able to look up. Now it is quite reasonable to 

 expect that a limit obtained practically with instrumental 

 appliances should fall short of a calculated theoretical limit, 

 but it is difficult to understand how it can exceed it. This is 

 certainly a point upon which more explanation is required. 

 The actual resolving power of the microscope, therefore, accord- 

 ing to Mr. Wright's theory, for a full cone, adopting the middle 

 formula above, viz., that derived by Lord Rayleigh from actual 

 experiment with a telescope, and employing the same wave- 

 lengths as those given in the tables in the " R. M. S. Journal " 

 for white and monochromatic blue light, viz., for lines E and F, 

 will be 



88,450 multiplied by the N.A. for line E. 

 and 95,880 „ „ „ F. 



But, as before remarked, the line E is too high up the 

 spectrum for visual purposes ; it will be better to take one 

 somewhat similar to that selected for my table in the " R. M. S. 

 Journal " for 1893, p. 17, then the limit will be 



85,630 multiplied by the N.A. 



As, however, a full cone in practice can seldom be used, 

 and because with a 3/4 cone spectra are present in the 

 outer annulus, the table which is given presently will still hold 

 good. 



To reduce this question to its simplest terms, the resolving 

 power of a microscope objective of N.A. TO, with a full cone, 



* My own experiments with telescopes on terrestrial objects, as well as 

 those ofDawes on Double Stars, agree with this value, 



