PROCEEDINGS OF THE SOCIETY. 221 



ill place by tlie brass setting (wliereby necessarily a greater or less part 

 of the periphery of the lens was rendered ineffective), should be fixed 

 by balsam to a thin plate of parallel glass rather larger than the lens, 

 the plate itself being attached by its jjeriphery to the brass mounting. 

 Professor Abbe, in reply, said that he had last summer tried the plan 

 with perfect success in some experiments made to test its efficiency, 

 and that several |ths had been so constructed. The device necessarily 

 introduced a very delicate additional point into the ojitical system 

 (the balsam film), and it would not be prudent to apply it where it is 

 not absolutely necessary, but it was the only way, in his opinion, by 

 which an objective of 1 • 35 numerical aperture ( = 128^'^ balsam angle) 

 could be made, and he hoped before long to accomplish this. 



Mr. Wenham said he should like to observe, with reference to the 

 proposed method of fixing the front lens, that Mr. Tolles about eight 

 years ago constructed an objective in which the front was fixed in a 

 similar way. 



Mr. J. Mayall, jun., said that he had examined the new -^^ oil- 

 immersion belonging to Mr. Crisp, and had measured the aperture with 

 the Abbe apertometer. He found it slightly less than the figure which 

 had been mentioned. As to flatness of field, it did not show that as 

 he should have expected to see it in similar powers by Powell and 

 Lealand or by Tolles. Whether flatness of field was a desirable 

 quality or not, he would not engage to decide ; it was, at any rate, a 

 quality much sought for by certain opticians. The lens was specially 

 designed for immersed objects or for objects in close contact with the 

 cover-glass, and when used on such objects gave fine definition. With 

 an immersion illuminator he had seen the striae of Amphlpleura pelln- 

 cida in balsam by lamplight with this lens more easily than with any 

 other he had examined. 



Mr. T. J. Parker read a paper by himself, " On some Applications 

 of Osmic Acid to Microscopical Purposes," illustrated by four slides. 



The President said that the Society was much obliged to Mr. Parker 

 for bringing the method described before them, as it appeared from the 

 specimens exhibited to be a very excellent one. 



Mr. Crisp said that some little misconception seemed to exist in 

 America as to the discussion at the October meeting on a unit of 

 micrometry. At the end of that discussion Dr. Edmunds made some 

 remarks on the Society's standard screw for objectives. This had 

 apj)arently been supposed to refer to a " Whitworth screw carefully 

 kept in the custody of the Society" as a standard for micrometric 

 measurements^ and was so referred to in an article by Professor 

 Rogers in the January number of the ' American Quarterly Micro- 

 scopical Journal.' As Professor Rogers, acting on this assumption, 

 proceeds to explain that " an absolutely perfect screw cannot be taken 

 as a standard, and hence this proposition of our friends abroad is 

 hardly feasible " (for reasons which we must all readily recognize), it 

 seemed desirable to make this correction. 



