president's address. 35 



and its value as a monochromatic illuminator was not con- 

 siderable. 



Yet it has for some long time been clearly seen that the ad- 

 vantages of true monochromatic light were great for special 

 purposes. It was made popularly manifest in detail by the 

 publication of Mr. Crisp's valuable "Aperture Table." 



Thus, for example, it is readily discoverable by it that while 

 a lens of JS'.A. 1*40 will, by calculation, in white light (line E.) 

 resolve 134,974 lines to the inch, the same lens employed with 

 pure monochromatic blue light (line F.) will resolve 146,305, 

 that is to say, will show to the skilled observer 11,331 more 

 lines to the inch. 



Again, an object glass having a numerical angle of 1'52 will, 

 in white light, reveal 146,543 lines to an inch ; but if illumi- 

 nated by pure monochromatic light, the same lens will reveal 

 158,845 lines to the inch, so that monochromatic light in this 

 case gives us an added power of 12,302 lines to the inch. 



Now by the hypothesis of dilfraction spectra, as explanatory 

 of microscopic vision, that means, what in reality it is, a prac- 

 tical increase of numerical aperture. 



But more, true monochromatic light really almost changes 

 an achromatic lens into an apochromatic one ; but the great 

 difficulty has been hitherto how to produce monochromatic 

 light which should be absolutely such, and yet be within the 

 reach of all, and under control as to its measure of intensity 

 when employed with high powers, so as to enable the worker to 

 employ a suitable corrected condenser. 



Sunlight is undoubtedly the purest and finest source, but we 

 so rarely have the privilege of seeing and using it that, if we 

 could obtain it from limelight, or still better, from strong 

 lamp-light, it would be a universal boon. 



Now, during the year Mr. Nelson has devised a very simple 

 and thoroughly practical piece of apparatus for obtaining a 

 good spectrum with either lamp-light t)r oxycalcium illumina- 

 tion, which allows the condenser to be used with every portion 

 of the spectrum at will, and gives light enough to work easily 

 with the violet end. 



With this, beautiful results are attainable with lenses which 

 the apochromatics had superseded; in other words it gives 

 them a renewed value. 



