36 president's address. 



But, as I pointed out when Mr. Nelson introduced this simple 

 instrument, I do not think that he claims for it what I believe 

 will be its largest result. 



As we saw last year, and have repeated to-night, we are 

 practically, for the present, at the limit of our possibilities by 

 mere increase of N.A. 



I believe it is held by practical opticians to be possible to 

 construct a lens with a N.A. of 2-0. But granted that the 

 working distance could be made sufficient, the mounting media 

 form, for the present, an insuperable difficulty. 



For this reason many skilled theorists and practical micro- 

 scopists are looking to the diminution of the wave-length of 

 light in specially constructed object-glasses as a means of their 

 further improvement. Dr. Czapski has in a comparatively 

 recent paper dealt with this aspect of the question.* 



The condenser, of course, would be inevitable, and all rays, 

 save those from a monochrome of the spectrum, would be 

 excluded; and these must have an intensity sufficient for all 

 purposes. But it may be certainly stated, as Dr. Czapski 

 shows, that by such means a lens with aN.A. of 1'40 would be 

 practically increased to 1'75 ; and so on with higher apertures 

 measured in white light when employed with the shorter wave- 

 lengths of the violet and blue rays.f 



But the special point of interest is that with object-glasses 

 of far less N.A. in white light we can come very approximately 

 towards 1'60 and yet employ mounting media that will not 

 destroy, but permit of the examination of fine organic tissues. 



Clearly, then, Mr. Nelson, with intelligent insight, has pro- 

 vided us with exactly the means we require for using with the 

 utmost advantage our best achromatics, and for getting the 

 greatest optical res alts possible from our most perfect apochro- 

 matics of large aperture. 



All this, of course, applies to the higher reaches of the micro- 

 scope and its work in research, and it would be a mistake indeed 

 if these were not included in the work of a society like this. 



It may be as well to note here that a solution is in use 

 by some microscopists which, although I have not tested it, I 



* " Probable Limits to the Capacity of the Microscope," p. 814, ' J.E.M.S.,' 

 part vi., 1891. ". Zeitschr. f. Wiss. Mikr.," viii., 1891, pp. 145-55. 

 t Ibid. 



