ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 131 



our finest objectives by means of Abbe's immersion condenser, Powell 

 and Lealand's truncated ditto, using the mirror in tbe axis, or by- 

 means of Tolles's transverse lens or equivalent means, using complex 

 reflecting prisms to reach tbe required obliquity of incidence, or by 

 swinging the mirror from tbe axis. But I want to see our illumina- 

 tion brought conveniently on a par with or beyond the apertures of 

 our finest objectives, so that we may readily test the value of the last 

 zone of working aperture. We have been told by Prof. Abbe what is 

 the theoretical resolving power of a numerical aperture of 1"5. I 

 want to see the matter demonstrated practically by means of a con- 

 venient and inexpensive condenser — one that will not require an 

 infinitesimally thin stage, or other elaboration of the mechanism of 

 our Microscopes. 



As to Mr. Nelson's observations on the ' true ' structure of certain 

 diatoms, I should have expected his rout on that point by ' Monachus ' 

 would at least have taught him that no amount of devotion to the 

 inspection of the surface of Amphijjleura pellucida would enable him 

 to decide the question of its ' true ' structure. If we really know 

 anything of the true structure of the finer diatoms, our knowledge 

 has been derived mainly from the comparison with coarser and 

 coarser forms, of which the structure has been made out, more or less 

 satisfactorily, by the examination of fractures or sections, and care- 

 fully tracing the correspondence with surface views — certainly not by 

 mere examination of the surface. As Mr. Nelson still clings to his 

 empirical views on the question of the determination of ' true ' struc- 

 ture, I must counsel him to read more closely ' Monachus's ' refutation 

 of his views. 



Oblique light is the most potent means we have of arriving at the 

 minimum visihile with the Microscope. I do not say that by it alone 

 can we arrive at true interpretations of minute structure ; nor, on the 

 other hand, can I agree with Mr. Nelson that we owe to its use all 

 our erroneous interpretations. It appears to me that errors of inter- 

 pretation are matters personal to the observer. If the observer will 

 insist on pledging himself to this or that view, regardless of the fact 

 that the whole of the necessary data may not be within his reach, 

 then, as I take it, he alone is responsible for the blunders he may 

 make ; and he should not blame oblique illumination, for, with suffi- 

 cient knowledge of the complexity of the conditions involved in 

 accurately diagnosing the structure of fine diatoms and of the means 

 at command towards the solution of the problem, he would give due 

 weight to every mode of illumination, and no more than its due 

 weight. Under such circumstances an oblique illuminator would be 

 an important factor, and I think it probable that Mr. Nelson's 

 dust-bin, if not already emptied, would yield up an odd contriv- 

 ance or two which the ' scientific microscopist ' would be glad to 

 possess." 



Mr. Nelson rejoined * that " F.E.M.S." is mistaken " if he thinks 



* Engl. Mccb., xl. (ISSi) p. 282. 



K 2 



