130 SUMMARY OF CURRENT RESEARCHES RELATING TO 



" F.R.M.S." tbiuks ♦ that in the interest of the Idstorij of micro- 

 BCopical science, Mr. Nelson should consider the service hitherto 

 rendered by " oblique illuminators " towards the improvement of the 

 Microscope. The category of oblique illuminators includes every 

 condenser devised, from the days of Descartes' gigantic parabolic 

 reflector applied at the nose-piece, down to the days of Powell and 

 Lealand's achromatic condenser. The former was afterwards modified 

 to its present form by Lieberkiihn, and is now termed the " Lieber- 

 kiihn," and Mr. Nelson gave it unqualified praise in his " demonstra- 

 tion " at the " Quekett " ; the latter, on the same occasion, he extolled 

 enthusiastically as " the finest condenser in the world." Every form 

 of condenser deals with oblique rays, and is, therefore, an oblique 

 illuminator, and as such has contributed its quota towards the 

 improvement of the Microscoiie. Nothing since the invention of the 

 Microscope has done more to cultivate the critical eye for excellence 

 in the optical construction than the striving to devise and utilise 

 condensers. Without condensers the importance of increasing the 

 apertures of the objectives might never have been discovered. 



" Mr. Nelson says that a new oblique illuminator of nearly 1 • 5 

 N.A. is ' not wanted,' I will endeavour briefly to show the use it 

 may be put to, bringing him in as a witness. 



Mr. Nelson has repeatedly admitted that the finest ' resolving ' 

 power of any objective is reached just before the obliquity of the illu- 

 mination is so great as to be beyond the aperture of the objective, i. e. 

 just before the dark field is reached. This is matter of common 

 experience, and I assume it to be agreed upon. Moreover, I refer 

 only to objectives of the best construction, which work accurately to 

 the limit of their aperture. It would appear, then, that Mr. Nelson 

 himself has not yet seen the finest resolving power of objectives of 

 1 • 43, 1 " 47, or 1*5 N.A. (the limit reached by Powell and Lealand in 

 the l/6ths made for the President and one of the Vice-Presidents of 

 the R.M.S.), because, if I am rightly informed, he has never had the 

 use of any oblique illuminator of higher N.A. than 1*4 (i.e. Powell 

 and Lealand's truncated oil-immersion condenser referred to by Mr. 

 Nelson on p. 240 of the current ' English Mechanic,' whence I quote its 

 numerical aperture). In order to obtain the finest ' resolving ' power 

 of such objectives, an oblique illuminator of 1'5 N.A. is therefore 

 required in spite of Mr. Nelson's negation, for with such a condenser 

 alone can we approximate to the ' dark field ' — the condition of 

 * finest resolution.' 



When Powell and Lealand issue their achromatic immersion con- 

 denser of 1 • 5 N.A., which has been on the way for many months 

 past, we shall, doubtless, be enabled to run through our present range 

 of apertures in a manner worthy of the splendid optical skill of these 

 opticians. But achromatism with such an aperture must necessarily 

 be costly. At present we are in the position of having objectives 

 with apertures beyond the reach of any recognized form of condenser. 

 We can, it is true, illuminate very near the limit of the apertures of 



* Eugl. Mech., xl. (1884) p. 264. 



