832 PROCEEDINGS OF THE SOCIETY. 



position. It embodied every feature likely to be required in an instru- 

 ment of such a type, and was not open to the objections that might be 

 urged against some well known stands supplied for the work in view. 



The thanks of the Society were accorded to Mr. Barnard for liis 

 exhibit. 



Dr. T. W. Butcher then read a paper on " The Structural Details of 

 Cose inodisc us aster -omphalus" which was illustrated with lantern slides. 



At the conclusion of his paper Dr. Butcher said that he wished very 

 particularly to express his thanks to Messrs. Watson and Sons, Messrs. 

 Angus and Co., and Messrs. Baker for the loan of Microscopes, various 

 lenses, etc., and to Mr. Powell who set up a beautiful lens with a very 

 perfect specimen under it. 



Dr. Spitta apologized for the brevity of his remarks, but he had to 

 leave in a few moments. He would show four slides. The first displayed 

 what was Mr. Nelson's focus of the diatom. The second, the " rosette 

 focus," as it has been called — the one in fact that appeared in most, if 

 not all, of Dr. Butcher's excellent photographs. As regards the mem- 

 brane which Mr. Nelson reported as having seen, Dr. Spitta approached 

 the subject in rather a different manner. First, if oblique light is used 

 with Mr. Nelson's focus, the image will almost (but not quite) form 

 itself into the same appearance as that obtained when using that illumina- 

 tion with the rosette focus. But the perfect image was only obtained 

 when this form of illumination was used with the latter (rosette) focus. 

 This, to his mind, proved that Mr. Nelson's focus was incorrect, and 

 that Dr. Butcher's was really the true layer of the diatom to focus. 

 This was shown in slide 3. To make sure that the effect obtained by 

 the use of oblique illumination was correct, it was a good plan to use 

 annular light, for if that showed anything approaching the same images, 

 it might be confidently believed no false images had been obtained by 

 the use of the previous method. This was shown in slide 4. Dr. Spitta 

 added that of course optical reasons forbade obtaining of so good an 

 image in detail with annular light as with oblique illumination, but of 

 that he had no time now to speak. The real point was — whichever focus 

 was tried, no membrane could be seen. He had visually tried several 

 apochromatic T V and the same type of lens (\ apochromatic) as Mr. 

 Nelson had used, in fact, all his photographs shown had been taken with 

 this very combination — yet nothing like a membrane could be seen. 

 With such a skilled observer as Mr. Nelson, the only explanation he 

 could offer was that the specimen used must have been of a different 

 kind from that ordinarily met with, for it was a mistake to think Mr. 

 Nelson's ^ apochromatic was of new design : Mr. Poser, of Carl Zeiss's, 

 had distinctly denied such to be the case — being only one out of the 

 ordinary stock. 



Mr. A. E. Brown said that, in corroboration of the correctness of the 

 photograph made by Mr. Nelson and exhibited by Dr. Spitta, it was 

 exactly that appearance of the structure of the diatom as found in a 

 Maryland deposit, which he always obtained with a Leitz T V oil-immer- 

 sion, when accurately in focus. 



Mr. Plaskitt suggested the intermediary structure as being the true 



