66 NOTES ON MICROSCOPICAL OPTICS 



A carefully computed objective of .85 N. A. will bear a full illumin- 

 ating cone on suitable objects, and can thus realise its fullest 

 resolving power. An objective of .95 with a condenser of .65 has 

 the resolving power of the mean, or of .80 N.A., and is thus actually 

 inferior except for freak resolutions with extremely oblique light. 

 Oil objectives over 1.30 or at most 1.35 N.A. are also of very 

 doubtful added value. 



In closing this section I will once more quote without comment 

 an anecdote of Fraunhofer, who received a complaint that a tele- 

 scope supplied by him, although giving magnificent images, displayed 

 certain hne scratches when examined with a magnifying glass ! The 

 reply sent by Fraunhofer is reported to have been : 



'■ We have constructed the telescope to be looked thrciigh, not 

 to be looked at." 



A few sentences may perhaps be added as to the prospects for 

 further improvements of microscopic performances. I have already 

 stated in the first section that there is a bright ray of hope with 

 regard to diminishing the curvature of field without loss of definition. 

 Advances in numerical aperture offer very little attraction. Abbe, 

 in my opinion, carried the N.A. too far rather than not far enough, 

 and I am not aware that any notable discovery has been achieved 

 with the few monobromide-immersion objectives of N.A. 1.60 which 

 he designed. 



The use of shorter wave-length, i.e., ultra-violet light, is a 

 little more promising. There would be none but technical difficulties 

 in the construction of lenses suitable for this work. But as only 

 very few microscopists would be likely to go to the trouble of 

 working in invisible light and of passing through a long apprentice- 

 ship in mastering the difficulties, apparatus of this description would 

 necessarily be extremely costly, as the whole expense of designing 

 and of constructing special tools would fall on a small number of 

 outfits, or possibly on only a single one. And there would still be 

 the grave drawback that the vast majority of objects would be 

 opaque to extreme ultra-violet rays, and would only yield black-and- 

 white outline pictures. 



The so-called ultra microscope does not represent any advance in 

 resolving power at all, but most decidedly the reverse. It is highly 

 valuable for the detection of very minute particles and of their 

 movements, which it achieves simply by intense darkground illumin- 

 ation, but the structure of the particles remains unrevealed, and 

 only that would amount to an advance in resolving power. The 

 seeing of these minute particles is, in fact, of precisely the same 

 kind as the seeing of stars subtending less than .001 second of arc 

 at nighb with the naked eye, the resolving power of wliich is of 

 the order of 60 seconds. 



