36 



THE AMEKICAN MONTHLY 



[February, 



Aperture and Eye-pieces. —The 

 communication of Mr. S. A. Webb, 

 which appears in another column, 

 contains a clear exposition of some 

 of the advantages of large angular 

 apertures for high-power lenses ; and 

 not many years ago, all of the as- 

 sumptions contained in his letter 

 would be fully endorsed by micro- 

 scopists. But now the case is differ- 

 ent. Without wishing to detract 

 from the real value of Mr. Webb's 

 letter, we deem it proper to point 

 out some errors for which, indeed, he 

 cannot be held responsible, since no 

 text-book in the English language con- 

 tains the results of the latest investi- 

 gations relative to the physics of 

 vision with the microscope. The 

 subject is too abstruse to be treated 

 in a condensed form, we would other- 

 wise have given our readers a 

 succinct analysis of it some time 

 ago. We intend to do so as soon 

 as we can devote the necessary 

 time to its preparation. Referring 

 now to Mr. Webb's letter, it should 

 be clearly understood that the " dif- 

 ferences of light and shade," which 

 he says " define and give us the 

 idea of form," are of no consequence 

 whatever, so far as definition of min- 

 ute textures is concerned ; for, in the 

 case of a diatom like P. angulatum, 

 for example, the rays which show 

 these differences of "light and shade" 

 caused by the partial absorption of 

 the light by the less transparent por- 

 tions of the diatom, may be totally 

 shut out from the ocular, but the fine 

 markings of the frustule will still 

 remain. Only the widely separated 

 and coarse parts of any object, speak- 

 ing comparatively,such as the outlines, 

 are defined in accordance with the 

 usually received explanation, by the 

 principles of dioptrics. The finer mark- 

 ings are imaged in a totally different 

 way, which we cannot now explain fur- 

 ther than by stating that they are only 

 defined when the illuminating rays 

 are decomposed by diffraction, and the 

 objective is of sufficient angular aper- 

 ture to afterward collect the rays and 



form them into spectral images. These 

 spectral images may be seen by re- 

 solving a coarsely marked diatom 

 with a ^-inch objective, then remov- 

 ing the eye-piece and looking down 

 the tube. 



These facts also lead to the con- 

 clusion that resolution is practically 

 independent of magnification, and 

 that there is a limit of magnification 

 for each objective, or rather for every 

 angular aperture, which enables the 

 eye to see every detail which the ob- 

 jective is capable of defining. As this 

 limit is not very high even for the 

 widest apertures — certainly not over 

 2,000 diameters — we may well ques- 

 tion the utility of very high eye- 

 pieces. 



o 



The American Society of Mi- 

 CROscopisTS. — The next meeting of 

 this society will doubtless be held at 

 Columbus. An invitation has been 

 received from that place, which will 

 probably be accepted. We will 

 endeavor to keep our readers fully in- 

 formed of all arrangements that are 

 made for the meeting. We hope none 

 of them will be deterred from prepar- 

 ing articles to read at the meeting on 

 account of the rule that was passed, 

 which forbids the publication of such 

 articles until they have appeared in 

 the Proceedings of the Society, for 

 we are confident that that rule will 

 not be enforced, even if it should 

 not be rescinded at the meeting. 



About Objectives. — Dr. Detmers 

 inquires, in his communication pub- 

 lished in another column, whether 

 the flagella of Bacterium termo have 

 ever been seen with any low-angled 

 objective. 



We are not aware that they have 

 been, in fact, we are quite sure that 

 they have not ; but one would infer 

 from the manner in which the ques- 

 tion was asked, that we had expressed 

 opinions at variance with the facts. 

 Thisis by no means correct, and the 



