PROCEEDINGS OF SOCIETIES. 187 



correct, as far as they go. Mr. Stodder figures an under-layer 

 (No. 1 in the ' Lens ') the existence of which I do not deny, though I 

 think his figure is not quite correct. I hope Mr. Stephenson will 

 soon publish his researches into the structure of Coscinodiscus Oculas 

 Iridis, as his mode of showing an inner layer is very instructive. 

 Two of Mr. Stodder's figures (3 and 4) — one, representing very irre- 

 gular hexagons, with indistinct markings ; and the other, irregular 

 black dots — appear to me to be affected by distortion, as well as in- 

 completeness. His figure 3, more regular hexagons with central bright 

 spots, is, I think, true under certain conditions of illumination, with 

 imperfect resolution. The central marks in these hexagons I conclude 

 result from the action of the lower layer. Fig. 5 in Mr. Stodder's 

 paper is not reconcilable with anything I have seen. I believe the 

 true formation to be much more symmetrical, and also more complex. 



I must leave microscopists who study chemical, as well as optical, 

 probabilities, to consider how far I am justified in thinking diatom 

 silica to be uniformly deposited in spherules. Many individual 

 diatoms show no spherules with means at present in use, but I know 

 no group in which they are not apparent, and as objectives and modes 

 of illumination improve, more and more spherules are seen. They 

 can now be traced in many species close to the limits of (present), 

 optical visibility ; I see no reason why they should be supposed not to 

 exist beyond it. 



I quite agree with Mr. Stodder in noticing that in many diatoms 

 (Coscinodisci, &c.) lines of fracture pass through the apparent de- 

 pressions, showing them to be the weakest parts. In most of such 

 cases the hexagonal borders appear to me composed of beads, and in 

 many cases the floors of the hexagons are beaded too. The lines 

 of fracture can often be traced to pass between the rows of these minute 

 beads, just as Mr. Wenham showed in the case of Pleurosigma, &c. 



Mr. Stodder thinks me wrong in objecting to the terms " areola?" 

 and " cellules " being applied to diatom markings. I do so because 

 I do not believe the diatom marks coincide in character with the 

 objects in other plants known by these names. 



I remain, &c, 



nth March, 1873. HENRY J. SLACK. 



PBOCEEDINGS OF SOCIETIES. 



Koyal Microscopical Society. 



King's College, March 5, 1873. 



Charles Brooke, Esq., President, in the chair. 



The President said he felt some little diffidence in occupying the 

 chair for the second time as their President ; he should not have thought 

 of doing so himself, and he must ask them to consider him as a stop-gap 

 (No ! No ! from a number of Fellows), because it unfortunately happened 



VOL. ix. p 



