PRESIDENT S ADDRESS. 



211 



results of labour associated with or attributable to the instru- 

 uient which we claim as our own may not be oat of place. And 

 at the outset we have no advance to report either in facilities for 

 the use of the objectives of great 'N.A. now in existence, nor in 

 the production of lenses of yet higher aperture. 



At present we are at a standstill. Mounting and immersion 

 media cannot at present be found which will enable us success- 

 fully to use a lens with a numerical aperture of 2 00, or even 

 1"60 ; and although there is much help afforded us in the use of 

 pure monochromatic light, enabling us to use achromatic lenses 

 more successfully, and both achromatic and apochromatic lenses 

 with increased aperture, there has been no special advance in 

 this matter during the year ; but it should be remembered that 

 this is no real proof that some splendid results may not yet be 

 obtained from the use of shortened wave-length represented by 

 apochromatic objectives. 



I have been experimenting on the entire group of my object- 

 glasses, as produced by the best English, European, and 

 American makers, for the last 26 years, and I certainly have 

 obtained most curious and even conflicting results ; but the sum 

 of these practically is, that I gather, what I suppose was 

 implied in the first presentation of the facts connected with the 

 scientific use of monochromatic light, viz., that to obtain the 

 highest results possible with it — to secure the largest theo- 

 retical and practical aperture with it — we want combinations 

 of lenses having mathematically adapted curves — in short, 

 objectives made to give the best results with a definite ray of 

 the spectrum just as the apochromatic objectives had to be 

 specially devised and figured to do their special work. Hence 

 it appears to me that we are not giving monochromatic light a 

 fair chance until we use, for high- powers specially, object- 

 glasses constructed to suit its refraction and dispersion. We do 

 not exhaust the new possibility presented by it by simply 

 showing its limitations when applied to existing object-glasses. 



I should be glad, indeed, if some one of our leading and com- 

 petent English opticians would address himself to this problem, 

 adapting lenses for use with the spectral ray that will give the 

 widest aperture in ordinary media, such as we can use without 

 violating the conditions which make the life of the organism 

 under examination impossible. There is a fine field open. 



