PROCEEDINGS OF THE SOCIETY. 799 



Mr . "Wesche noticed that the glass rod was inclosed in a brass tube, 

 and inquired if this was in order to increase the reflection. 



Mr. Beck said that this was merely for the purpose of protection. 



Mr. Rousselet asked if the rod was large enough to enable the lamp 

 to be used for low powers and dark-ground illumination. 



Mr. Beck said that the lamp worked very well with low powers down 

 to 1 or lj in., but if wanted for low-power work it should be used with 

 its condenser. 



The thanks of the Society were voted to Mr. Beck for his exhibit. 



A paper " On the Recent and Fossil Foraminifera of the Shore-sands 

 of Selsey Bill, Sussex "—Part IV., by Mr. E. Heron-Allen and Mr. A. 

 Earland, was read by Mr. Heron-Allen, who expressed his regret that 

 Mr. Earland was unable through illness to be present on that occasion. 

 The subject was illustrated by about ninety extremely fine lantern slides 

 shown upon the screen, and by a large number of mounted specimens 

 shown under Microscopes on the table. The indebtedness of the authors 

 of the paper to Mr. A. E. Smith, who had prepared the photomicrographs 

 from the mounts, to Mr. J. A. Lovegrove, who had prepared the lantern 

 slides illustrating the objects, and to Mr. H. F. Angus, for arranging 

 the exhibition under the Microscopes and upon the screen, was duly 

 acknowledged. 



Mr. Edmund J. Spitta said that as he knew so little about 

 Foraminifera he could not say anything about these little " beasties " as 

 Mr. Heron-Allen had called them, but he should like to convey his 

 congratulations upon the excellence of the photomicrographs shown. 

 These, the speaker said, were taken, he understood, by his old friend 

 Mr. Smith, the slides being made by Mr. Lovegrove. Such large and 

 thick objects as those in question offered difficulties peculiar to them- 

 selves and of no small moment, for be it understood it was absolutely 

 impossible to show them in complete focus on account of their great 

 thickness. The art of the operator was in the first place to settle which 

 plane was the best to have in absolute focus, and in the second, to 

 decide upon how much depth of focus was permissible. If the iris- 

 diaphragm was closed too much for this purpose, all manner of dif- 

 fraction phenomena presented themselves and the resulting photograph 

 showed all sorts of false images which would lead astray those who 

 looked upon the photograph as representing the real truth and nothing 

 but the truth. Besides this too, seeing that closing the iris-diaphragm 

 reduced the N.A. of the objective in use, so it reduced its resolving 

 power also, and in consequence the photographer would not be able to 

 show all the minute structure of the specimen in the plane of focus he 

 had selected for that purpose. It would be understood then that two 

 evils were apt to result by attempting to produce a too great depth of 

 focus, the introduction of false details — details that did not exist but 

 were optically caused ; and the omission of fine details that perhaps 

 were really present, but were prevented from being shown by the undue 

 lowering of the numerical aperture. On the other hand, it must not lie 

 forgotten, the photographer was compelled to get a little depth of focns 

 by closing his iris-diaphragm a trifle, for otherwise the object looked 



