THE PRESIDENT'S ADDRESS. 



31 



pass is only 12| per cent, less than the area of the central por- 

 tion illuminated by the 3/4 cone. The numerical values are: — 

 Darkened annulus ... ... ... 43' 75 



Illuminated central portion ... 56 25 



100 



Because in practice we are bound to use a 3 4 cone we shall 

 therefore have an image compounded of a true image in the 

 central three-quarters of the whole aperture, according to Mr. 

 Wright's theory, and an Abbe diffraction, or " true false " 

 image, in the peripheral annulus, according to my nomenclature. 

 The resolving limit will therefore, as stated above, agree with 

 this table from the " R. M. S. Journal." 



Table of Resolving Powers in Lines to an Inch with 3 [4 Gone 

 of Direct Illumination. 



The above table agrees remarkably well with results actually 

 obtained with the best lenses, and to show that this is so the 

 following table gives the actual resolutions made on diatoms in 



