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PROCEEDINGS OF THE SOCIETY. 
Meeting of 18 th June, 1890, at 20, Hanover Square, W., 
Frank Crisp, Esq., B.A., LL.B., V.P.L.S., in the Chair. 
The Minutes of the meeting of 21st May last were read and 
confirmed, and were signed by the Chairman. 
The List of Donations (exclusive of exchanges and reprints) received 
since the last meeting was submitted, and the thanks of the Society given 
to the donors. 
From 
Specimens of Patella pelhicida Mr. W. B. Strug nail. 
Flint-glass slide of mixed Diatoms Dr. H. Van Heurck. 
Mr. J. May all, jun., said the flint-glass slide received from Dr. H. 
Yan Heurck, Director of the Botanical Garden, Antwerp, had been 
forwarded to replace the slide received from Prof. Abbe with the new 
1/10 objective of 1*6 N. A., which slide had unfortunately become so 
deteriorated by partial crystallization of the extra-dense mounting 
medium that it had not been possible to test the objective satisfactorily. 
The new slide was stated by Dr. Yan Heurck to contain several fine 
specimens, and it was hoped that it would facilitate the preparation of 
the report on the objective. 
Mr. Mayall mentioned in explanation of the delay in bringing 
forward the report on the new objective, that before the committee — con- 
sisting of Dr. Dallinger, Mr. E. M. Nelson, and himself — met officially 
to examine the objective, it had been agreed to support the report by the 
production of photomicrographs of the various objects used as tests. 
With this view, Mr. Nelson and himself met, and after devoting several 
hours to the examination of sundry diatoms, on some of which the 
definition was remarkably good, it was decided to photograph the best 
fractured valve of P. angulatum observed on the slide. They were, 
however, disappointed to find that the visual and actinic foci w*ere not 
coincident, which fact was demonstrated (1) by the visual image that 
had been accurately focused appearing in the photographs wholly 
indistinct ; (2) by Mr. Nelson’s guessing what focal allowance to make, 
so that the out-of-visual-focus-image came out very nearly sharp in the 
photograph ; and (3), in further proof of the point, a coarsely marked 
diatom, as to the focus of which there could not be a doubt, was first 
accurately focused, then photographed, the photograph appearing ' 
wholly indistinct. The focus was then altered by Mr. Nelson by an 
amount which he roughly estimated would suffice. The visual image 
was then wholly indistinct, but the photograph produced of it was very 
nearly sharp — very nearly as sharp as the image could be seen in the 
Microscope when accurately focused. Under these circumstances the 
possibility of producing critically good illustrative photographs with the 
objective as it then was seemed very doubtful ; it was therefore decided, 
with Dr. Dallinger’s consent, to communicate the facts to Prof. Abbe, and 
by his request the objective was returned to Jena. After the lapse of 
