146 
PROCEEDINGS OF THE SOCIETY. 
either side of the valve there is any even boundary of dividing bars to 
produce this figure, no matter how realistic the picture. We have seen 
from the photographs that on the outer side the appearance is produced 
by a swelling out of the bright patches when the focus is depressed, and 
on the inner it seems to be due to the short bars — or intercostals — being 
disposed around the dark fields in such a manner that the intermediate 
bars are set at an oblique angle to the top and bottom ones. On this 
question, however, I speak with all diffidence, as it seems to be one on 
which we require more light in the shape of N.A.” 
Mr. Nelson said it became an interesting question, what were the 
conclusions which they must logically come to if these intercostals were 
realities ? The matter was one which certainly required some further 
light, unless they were to experience an entire alteration of their opinions 
of the Abbe theory. By means of blackboard diagrams he then showed 
what ought to appear according to theory and what seemed to be seen 
in practice. He expressed his suspicion that there was probably some 
truth in each, and that it would be found that they had a very strong 
structure of 50,000 and a very slight structure of 100,000, and that as 
regarded the spectra they had really the six strongly shown, but the 
others were also to be detected very faintly outside. 
Mr. J. E. Ingpen said he could quite corroborate what Mr. Nelson 
had said — speaking from memory as to a photograph of a diffraction 
grating which had strong lines well photographed, but owing to some 
circumstance there was a very faint indistinct line between them, appa- 
rently not capable of giving a spectrum. 
Mr. T. Comber said he had been studying the structure of diatom- 
valves for some little time, and in one respect at least he could confirm 
what Mr. Smith had said, for he believed that all the specimens of 
Pleurosigma had valves consisting of two layers. He had brought with 
him two photographs to show the existence of these two layers, as they 
could there be very clearly seen, the one projecting beyond the other. 
The other parts mentioned in Mr. Smith’s paper he had not been able 
to see himself, although he had tried his best to do so. Mr. Smith said 
that they could not be seen in the case of diatoms mounted in styrax or 
other highly refractive media — but that he could see them in specimens 
mounted dry. He thought if this was so, they were very likely to reach 
conclusions which might be very misleading, supposing that diatoms 
believed to be dry became damp — as they were very apt to do. 
The thanks of the meeting were voted to Mr. Smith for his com- 
munication and exhibition. 
The President said it would no doubt be remembered that some 
time ago the Society sent to the Chicago Exhibition some very beautiful 
photomicrographs supplied by Mr. Nelson, Mr. White, and Mr. Pringle. 
He believed that these exhibits had obtained a medal ; and though they 
had at present received no official intimation of the fact, the news had 
come to them in a way which was so thoroughly characteristic of their 
friends on the other side of the water, that they might like to hear the 
communication read. 
Dr. Dallinger then read the circular, greatly to the amusement of 
the Fellows present. 
