ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 813 



researches on the valve of Pleurosigma, and claimed to have discovered 

 that what up to that time had been cousidered a single plate of silex 

 was really built up of two or three layers of structure. I also claimed 

 to be the first to call attention to this fact, but this claim 1 must now 

 withdraw, for the simple reason that I find on page 680 of the Journal 

 of this Society for 1879 the following passage from a paper by Herr 

 Grunow — with additional notes by Mr. Kitton — on the Diatomacese of 

 the Caspian Sea : — 



Speaking of Pleurosigma attenuatum and P. Jiippocampus, Herr Grunow 

 says : — " The structure of these allied forms under high powers appears 

 very similar ; between the strongly marked lines of beads faint outlines 

 of other beads may be seen. Whether these delicate puncta belong to 

 a second valve or are an optical delusion must remain for the present 

 undecided; it is certain, however, that the valves of Pleurosigma are 

 composed of two layers, which separate when acted upon by long boiling 

 in acids." And then between brackets, I suppose by Mr. Kitton, " I have 

 seen this in P. angulatum." Then follows this note by Mr. Kitton : — 

 " The faint markings here alluded to have been seen by other observers. 

 It is most probable that the valves of Pleurosigma have a similar struc- 

 ture to many other diatoms in possessing what I call secondary valves, 

 which in some genera are like, and in others unlike the primary valve." 



The above passages show how remote the chances are of any sint^le 

 individual being the sole discoverer of any new fact, whether important 

 or trivial; and although the only positive evidence given here is the 

 separation of the layers by boiling in acids, it is enough to bar my claim 

 to be the first to call attention to the compound structure. I think, 

 however, I may still claim to be the first to figure the structure of the 

 diff"erent layers, and am pleased to feel that my attempts in this direction 

 will derive additional weight from being corroborated by the testimony 

 of two such eminent observers as Herr Grunow and Mr. Kitton. 



It is almost necessary to apologize for bringing this subject before 

 you to-night, as for some reason the study of diatoms in the present 

 day is almost a discredited one, and the microscopist who indulges in it 

 is looked upon as notliing better than a trifler in science. But I think 

 this stigma is an unjust one if we look at the important part the 

 resolution of diatoms has played in the development of the modern 

 objective, and thus placed in the hands of microscopists generally an 

 efficient instrument of research, without which many pages of Nature 

 must have remained a sealed book. The study of diatoms has also its 

 value — and with many its chief value — in their being one link in the 

 great chain of existence ; but it is purely from a brass-and-glass point of 

 view I wish to approach them to-night, and using them as a standard 

 of value, try to prove by the results of my investigations on the Pleuro- 

 sigma valve, how much further it is possible, by the use of the new 

 optical glass and proper methods of illumination, to push our researches 

 into the nature of all minute structures. 



Practically, the resolving power of our objectives on lined objects 

 had reached its maximum before the advent of the new glass. The 

 Ampliipleura pellucida marks now, as it marked then, the finest known 

 regular structure of any regular object. There was nothing further, 

 then, to be gained in resolution, but possession of one of the new 

 apochromatics, with its entire absence of colour, soon convinced me that 

 it possessed a power of separating different layers of structure altof^ether 



