132 SUMMARY OF CURRENT RESEARCHES RELATING TO 



that I do not value oblique illuminators as matters of history in the 

 development of the Microscope. It was merely in their use as 

 instruiiKiuts of modern scientific research that I condemned them. 

 Condensers and Lieberkiihus cannot be justly called oblique illu- 

 minators, though they may be used as such. I cannot agree with the 

 statement that ' oblique light is the most potent means we have of 

 arriving at the miuimmn visibilc with the Microscope.' So far as I 

 know, the smallest object which lias been publicly exhibited is the 

 flagellum of a Micrococcus which I showed at the Q.M.C., and by 

 invitation at the soiree of the R.M.S. The length of the double 

 micro-organism was only 1/12,000 in., the flagellum was barely half 



that length. Now if we take as a maximum estimate -:; — = 1/6, 



length 



we shall have 1/144,000 in. as the thickness of the filament. This 



ratio is probably greatly in excess of the truth. This object is only 



visible with direct light : oblique light completely obliterates it." 



" F.K.M.S." in reply * considers that Mr. Nelson " lias involved 

 himself in the following paradox : — He considers that the diatome- 

 scope, which provides oblique rays in one azimuth, is an oblique 

 illuminator ; whereas the Lieberkiihn or a condenser which provides 

 oblique rays in all azimuths, is not an oblique illuminator. In other 

 phrase : Light incident in one azimuth is oblique ; in all azimuths, 

 not oblique ! I will leave him to explain the paradox. 



He does not agree with my remark that ' oblique light is the 

 most potent means we have of arriving at the minimum visihile with 

 the Microscope,' and as an example of what he regards as the 

 minimum visihile, he cites a flagellum of a Micrococcus estimated at 

 1/144,000 in. in thickness, 'which is only visible with direct light; 

 oblique light completely obliterates it.' 



On this I remark, firstly, that I think his direct light, if critically 

 examined, will be found to consist chiefly of oblique light. 



Secondly, by way of parallel example embodying the opposite 

 view, I cite one of the most prominent items of what is generally 

 admitted to be ' modern scientific research,' the original discovery of 

 the flagellum of B. termo by the eminent microscopist Dr. Dallinger.'f 

 This discovery was made by the use of the most oblique light 

 obtainable by the recognized condensers of that date, a method of 

 research utterly condemned by Mr. Nelson. That in Mr. Nelson's 

 hands the flagellum should be ' obliterated ' by the same kind of 

 illumination by which Dr. Dallinger first discovered a similar 

 flagellum, is another paradox, which I leave for his consideration. 

 I note, in passing, that Dr. Dallinger, in referring to the oblique 

 illuminator he employed, wrote that it had ' the advantage of throwing 

 the light in only from one direction [in azimuth].' Now, singular as 

 it may appear, Mr. Nelson condemns the diatomescope for possessing 

 the qualification commended by Dr. Dallinger. 



* Engl. Mech., xl. (1884) p. -299. 



t Mod. Micr. Journ., xiv. (1875) pp. 105-8. 



