234 



Transactions of the Society. 



as phenomena illustrating a distinct idea of the functions of the 

 microscope. For there is no want of optical curiosities among 

 microscopists ; and I take no interest in bringing forth more of 

 such. Please make any use of my explanations you like. 



With my best regards, I remain, 



Yours truly, 



(Signed) 



E. Abbe. 



Note by Professor Cheshire. 



By making use of a simple graphical method for determining 

 the angular distribution of the diffracted beams produced by a 

 grating, which I have used for many years, but which, so far as I 

 am aware, has never been published, the proof of the equation for 

 the resolving power of a microscope can be presented in a simple 

 and convincing way. The method is as follows : — 



2«-^O.S. 



/ii. o.s 



3^»0.S. 



Graphic method of determining the angular dispersion of the 

 diffracted beams produced by a grating. 



. ' Let B be a grating, immersed in a medium of refractive index 

 n, upon which homogeneous light falls at an angle of incidence a. 

 With the point of intersection e of the direct ray with the grating 

 as centre, draw a semicircle with radius equal to unity, and from 

 the point of intersection of the direct beam with this semicircle 



