1068 PKOCEEDINGS OF THE SOCIETY. 



he used for the purpose, whence it was clear that the prisms were similar in 

 form, though apparently that of M. Nachet was much smaller than those 

 employed by Prof. Harting, of which Mr. Crisp possessed two. M. Nachet 

 had also sent over a prism such as he used in his tri-ocular Microscope, and 

 it would be seen that it was a very complicated one, the construction of 

 which would probably puzzle many persons. He had also forwarded one 

 of his prisms used in the bi-ocular form. These prisms were submitted for 

 the inspection of the meeting. 



Mr. C. D. Ahrens' Microscope, with three body-tubes and three objec- 

 tives, was exhibited by Mr. Crisp {ante, p. 799). 



Mr. J. Mayall, jun., said that in devising this Microscope he did not 

 think that Mr. Ahrens had any scientific end in view, his idea merely being 

 to have something which would serve the purpose of three Microscopes in 

 one, to exhibit to the public on festive occasions at Hampton Court and 

 similar places. 



Mr. Crisp said that since their last meeting they had had a correspond- 

 ence with Dr. Van Heurck on the subject of the remarks made at the January 

 meeting by Mr. Mayall and Mr. Beck as to the photographs of AmpMpleiira 

 pellucida made by Dr. Van Heurck, the suggestion being that the background 

 of the photographs had been painted out. Another set of photographs had 

 now been sent over which had not been manipulated, and which, Dr. Van 

 Heurck claimed, showed all he had formerly represented. 



Dr. Van Heurck's letter was as follows : — 



"Basing himself on what was said by Mr. Mayall at your January 

 meeting. Dr. J. D. Cox stated, at a meeting of the New York Microscopical 

 Society, that my photographs had undergone notable alterations. 



As I cannot allow such assertions to pass uncontradicted, casting doubts 

 on the accuracy of my observations, I have sent to New York some further 

 prints of my photographs, duplicates of which I send to you. It will be 

 readily seen that these have not undergone the slightest alteration. 



I specially call your attention to the longitudinal lines of the Amphi- 

 pleura pellucida, which are vastly more difficult than the beads. The 

 undulated nature of these striae, so different to diffraction lines, shows clearly 

 that they are real longitudinal lines." 



Mr. J. Mayall, jun., said he was sorry that Dr. Van Heurck should have 

 objected to his criticism, but he must insist that he was fairly entitled to 

 criticize the photographs which he saw on the previous occasion ; and when 

 it was said that no diffraction lines were visible, he could hardly do other- 

 wise than point out that the natural background on which they were said 

 to be projected had been blocked out, leaving the field quite white, and 

 consequently destroying the natural outlines of the diatoms. He considered 

 the photographs to be excellently taken, but he objected strongly to the use 

 of an artificial background, which gave fictitious outlines to the objects. 

 He considered all such manipulations as seriously interfering with the 

 scientific value of photomicrographs. 



Mr. Enock's preparation of the Hessian fly, and also of its parasite 

 (Semiotellus destructor) was exhibited. 



Prof. Bell said of course they all devoutly hoped that the parasite might 

 increase and multiply abundantly. The fly itself seemed of late to have 

 been making as much stir in this country as the Colorado beetle did some 

 years ago. It was a very important matter to know that the adult form 



