128 T. F. SMITH ON DIATOM STRUCTURE. 



been using these plates as a test for my own j\" oil immersion, 

 and, allowing for the difference of N.A. between 1*25 and 143, 

 was at first satisfied to find that my glass gave on this diatom 

 the same indications of structure. On a second sitting, how- 

 ever, I discovered a layer of eye-spots with thickened edges 

 on the under side ; and my experience tells me that a structure 

 of hexagonal cells is always built upon the top of these, and 

 that in spite of appearances this is the structure in this case. 

 The blacker dots shown in the figure in the shape of hexagons 

 I take to be spurious, but the lighter ones in the centre not, 

 being the perforations in the usual covering membrane. 



I grant appearances are, at first sight, against me, and it 

 may seem rash of me to pit myself and my humble glass against 

 the best manipulators in England, working with the most 

 advanced optical appliances ; but with all deference, I must 

 still stick to my opinion that straight-sided cells and not black 

 dots is the true structure. 



This is no new question, as in the April number of the 

 " Monthly Microscopical Journal," for 1871, is a photograph of 

 a Coscinodiscus, by Dr. Woodward, of America, and in the letter- 

 press he says : " On one side the hemispheres are quite a little 

 distance apart. On the other side they are crowded together, 

 producing the spurious appearance of a hexagonal framework 

 with little spherical beads at the corner." After this prompt 

 suppression of the hexagons by our then greatest authorit}^ on 

 diatom structure, I find no mention of them until December 

 4th, 1872, when Mr. Slack expressed a timid opinion that the 

 hexagons were real, but in order to confine this new teaching 

 within the limits of the orthodoxy of the day, decided that they 

 were beaded. But what I particularly want to call your atten- 

 tion to, as bearing on the structure of Aulacodiscus Kittonii, is 

 ;i discussion that also took place in what Mr. Crisp so aptly 

 calls " the dark ages," that is, before the advent of oil immer- 

 sion object glasses, on the structure of Coscinodiscus oculus 

 iridis, in which Mr. Slack expressed an opinion that each side 

 of the hexagons was made up of two rows of little beads. Mr. 

 Slack, no doubt, saw beads, as at that time he and nearly all 

 microscopists saw beads in nearly all diatom structure ; but 

 the reason for seeing them was that the true structure was just 

 outside the grasp of the object glasses of that day, and when 



