PROCEEDINGS OF THE SOCIETY. 
431 
diaphragms rotating beneath the stage. The more modern arrangements 
of central stops to be used in conjunction with some form of condenser, 
or with a lieberkiihn, were far preferable. 
Prof. Bell read an abstract of a paper contributed by Surgeon Y. 
Gunson Thorpe, R.N., on “ Some New and Foreign Rotifera ” found on 
the West Coast of Africa, and belonging to the genera Trochosphsera, 
Floscularict , and others. The paper had been submitted to their late 
President, Dr. Hudson, who regarded it as one of great interest, 
and strongly recommended the Society to print it in extenso with the 
figures. 
Mr. E. M. Nelson referring to the subject of his paper read at the 
meeting of the Society in March last, exhibited two forms of bull’s-eye 
condenser, one of which was made like Herochel’s aplanatic, and the 
other was a new and simpler form than he had previously described, 
being made of two plano-convex lenses, the mounting of which was also 
peculiar, and what he considered to be the most useful method. This 
condenser seemed to answer its purpose admirably, the amount of 
spherical aberration being only about one-fifth of that which existed in 
the old form. 
Mr. Nelson also read a paper entitled “ Further Notes on Diatom 
Structures as Test Objects,” which he illustrated by photographs. 
The President said the photographs appeared beautifully clear, and 
would, he thought, be found of much interest as bearing upon the points 
to which Mr. Nelson had particularly drawn their attention. 
Mr. C. Haughton Gill’s paper “ On the structure of certain Diatom 
valves as shown by sections of charged specimens ” was read, the subject 
being illustrated by photomicrographs. 
The President thought Mr. Gill’s experiments were of great interest ; 
but the subject was so new that discussion could hardly take place, 
especially in the absence of the author, which he regretted to learn was 
due to illness. 
Mr. Mayall said the problem Mr. Gill had endeavoured to solve was 
as to the existence or not of cellular structure in diatoms extending 
through their substance, and this he sought to demonstrate by making 
chemical depositions of a more or less opaque character, which would 
probably fill up the cavities sufficiently to be plainly distinguished by 
the Microscope. He (Mr. Mayall) had some time ago translated a paper 
which appeared in the Proceedings of the Belgian Microscopical Society, 
on a process of preparing sections of diatoms by grinding portions of 
Cementstein from Jutland. The sections obtained in this way were 
imbedded in the material forming the matrix of the stone, and were filled 
up by the same material, showing to some extent the points which Mr. Gill 
had reached in another way. He thought that Mr. Gill’s observations 
were of still greater interest because he had not merely taken a natural 
formation, but had deliberately experimented with the definite purpose 
of testing a special point, thus applying to Microscopy what Herschel 
would have termed an “ experiment of inquiry ” — a direct questioning of 
nature on a point that had hitherto been regarded as almost beyond the 
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