ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 853 



Designation of Eye-pieces.* — At the Elmira meeting of the 

 American Society of Microscopists, Dr. R. H. Ward, the chairman of 

 the committee on eye-pieces (ante, p. 103), reported, that " all manu- 

 facturers but one had agreed to designate their eye-pieces by their 

 focal-lengths, but no agreement had yet been made as to the diameter 

 of the tubes." The committee was continued for another year. 



Objectives of small and large Aperture.t — The Rev. W. H. 



Dallinger writes on this subject as follows: — "No one has appre- 

 ciated or found more pleasure and profit in the use of the large angles 

 with which our lenses have been more and more perfectly provided 

 for the last ten or twelve years than I have. As they have been 

 produced I have obtained them each and all, that had any real value, 

 whether produced in this country, the Continent, or America, and in 

 some cases I have incited certain English makers to produce certain 

 special formulas during that time. But while I have used all lenses, 

 from the £ to the -^, constantly during this time, what work I have 

 done could never have been accomplished if I had only had lenses 

 with large angles to work with. Much that had been done could 

 never have been done without them ; but the work, as a whole, could 

 never have been done at all if only such had been at my disposal. Hence 

 I have, in all my special working powers, three lenses of the same 

 power, and in some cases four, and each of them, in following out the 

 details of a life-history of an organism, say of the ^oVo *° * ne 60^0 °f 

 an inch in length, is absolutely needed, and its place cannot be 

 supplied by the other. Thus, I have two ^ths, one having a very 

 low angle, and the other as great a numerical aperture as an oil-immer- 

 sion can provide when worked by the best makers. In the 7 Vth> 

 I have but one lens, a medium angle, because it was intended only 

 for general work and, mainly, central illumination. I have, however, 

 three ^ths, four -^ths, and so on ; and I know exactly what each 

 will do, and no more attempt to get the work of one out of the other 

 than the maker of them would attempt to get their several results by 

 grinding them to the same formulae. 



I talked this matter over in detail, pointing out results, six years 

 ago with some leading experts ; and although, during two or three 

 years, many have thought that Abbe's mathematics and views were 

 adverse to this view of mine, I felt convinced by reading between the 

 lines of his papers, and remembering their special object, that it was 

 not so. Still Dr. Carpenter was good enough to get a detailed view 

 of my experience and opinion before publishing the last edition of 

 ' The Microscope,' and he has in his preface and throughout the 

 volume, given in effect my views, which now the unmistakable decla- 

 rations of Abbe coincide with and confirm. The homogeneous lenses 

 have given me splendid results, some of which will shortly be 

 published ; but no immersion lens of any kind could be used to work 

 out to the end an organic life-history — that is, if it involved life and 

 movement, because the object being in a limited area, and possibly 

 in fluid, the fluid under the cover does (when the movements of 



* Amer. Mon. Micr. Journ., iii. (1882) pp. 175 and 171. 

 t North. Microscopist, ii. (1882) pp. 288-9. 



