762 PEOCEEDINGS OF THE SOCIETY. 



bottom separated by a considerable distance was what resulted from 

 increase of magnifying power. This was, therefore, quite apart from 

 any increased resolving power in plan, a very important matter when it 

 was a question of showing a particular plane in any composite object. 



A further paper by Mr. Merlin, entitled " A Note on a New Pris- 

 matic Ocular," was also read by Dr. Hebb, the diagram in illustration 

 having been previously drawn on the board. 



Mr. Beck said the novelty about this contrivance appeared to be the 

 curved surfaces. Chevalier, of Paris, had made one of the same kind 

 with flat surfaces, and it had since been made with what is called a roof 

 prism, which gave a perfect erection, both right and left and up and 

 down. Such a prism is only serviceable when the Ramsden circle is 

 situated at a considerable distance from the upper surface of the eye- 

 piece. 



A note by Mr. E. M. Nelson " On a New ^ inch Semi-apochromatic 

 Objective of • 74 aperture, computed by Mr. Conrady," was read by the 

 Secretary. 



Mr. Spitta said that he had had the privilege of seeing the objective 

 in its workshop mount, and that he could corroborate all Mr. Nelson 

 said. As, however, he noticed this gentleman had not mentioned any 

 use of the Abbe plate, he rose to say that the performance of the com- 

 bination in question with this — the severest of all tests — was very 

 satisfactory : indeed, the correction of the outer zone, which was always 

 the weakest in most sixths, was distinctly an advance. Of course the 

 aperture of this new sixth was not so large as that of the holoscopic 

 series having the same focal length, but notwithstanding this, it resolved 

 the dots in Nitzschia scalaris with direct light exceedingly well, far 

 better, he was bound in common justice to say, than some, although he 

 did not wish to say all, other examples of the optician's craft (of similar 

 focal length) which had come before his notice. The long working 

 distance was, perhaps, the leading feature of the new objective ; he had 

 never known one so great, and, as Mr. Nelson had found, it amounted 

 to at least a millimetre in length, which might be called positively 

 phenomenal. He should like to be permitted to congratulate both the 

 computer and the manufacturer. 



The Chairman said it was a matter of considerable importance to 

 have a lens with a long working distance, because, when examining a 

 hanging-drop preparation, it was most desirable to be able to focus from 

 the top to the bottom of the drop. A lens of this kind was quite an 

 acquisition to working bacteriologists. 



A Paper by Mr. Alfred Letheby "On Systematic Exposure with 

 Transmitted Light in Photomicrography " was taken as read. 



The Secretary said they had received from Mr. F. H. Baker and 

 Mr. J. D. Macphail some material, consisting of Diatomaceous earth 

 and Radiolaria, for distribution to any Fellows wishing to have any. 

 Application for samples of these should be made to Mr. Parsons. 



