510 



MICROSCOPY. 



(unity), and a resolving power limited to 96,400 

 lines to the inch. All powers above these are 

 in excess of the possibilities of a dry lens. For 

 obvious reasons, the numerical aperture in- 

 creases slowly, in all cases, as the objective 

 approaches 180, the last few degrees of angu- 

 lar aperture being of small relative value. The 

 following table gives a selection from Prof. 

 Abbe's elaborate computations of these data:* 



The question as to the choice of moderate or 

 extreme apertures for objectives is still open 

 and somewhat evenly disputed. Dr. Carpenter 

 ("TheMic.," p. 200; "End. Brit.," 9th ed., 

 1883, xvi, p. 269 ; also, his address at the Moat- 

 real meeting of the A. A. A. S. in 1882, "A. 

 M. M. J.," 1882, p. 161 ; "Mic. News," 1883, 

 p. 29) and a large following of conservative 



judges still hold to theformerView not doubk 

 ing the value of large aperture, but believing 

 that it should accompany higher powers, and 

 that to a 1-inch, for instance, should not be 

 assigned the aperture and work of a th, nor 

 to a -^jj-th that of a -|th ; and this view is cor- 

 roborated by the mathematical computations 

 of the relation of aperture to power by Prof 

 Abbe (" J. K. M. S.," 1883, p. 790). On the 

 other hand, a large number of experienced 

 persons prefer large angles even for biological 

 work ; some claiming that the highest attain- 

 able angles are the best for all uses and powers. 

 The following table shows in comparison the 

 apertures that have been suggested by the above 

 writers, and those now made by the principal 

 American makers, the angular apertures being 

 given except where na is stated. The figures 

 in the Abbe column apply only approximately 

 to the adopted nomenclature by inches ; and 

 they are given, not as a working standard, but 

 as a limit required by theory, and which will 

 be more or less exceeded in practice. The last 

 three columns represent the writer's selection 

 of three ideal series of objectives, of low, me- 

 dium, and high degrees of completeness and 

 efficiency. There are few persons whose mi- 

 croscopical work could not be well done by 

 the last-named series ; though a few special- 

 ists, like the Rev. W. H. Dallinger, in his 

 unique researches among the minutest forms 

 of life, employ a great number of lenses which 

 would be practically duplicates in other hands. 



APEKTURES. 



Variable objectives of low power, mostly 2 

 or 3 to 4 or 5 inch, changing power gradually 

 by the separation of the lenses by screw-collar 



* For further account of these researches, which constitute 

 one of the most important and revolutionary contributions 

 ever made to theoretical microscopy, see the elaborate papers 

 by Prof. Abbe, Mr. Stephenson. and Mr. Crisp ( u J. K. M. S. " 

 1879, p. 639; 1881, pp. 303, 888; 1882, pp. 800, 460; 1883, p. 

 790 ; 1S84, p 20). 



movement, were early made by Mr. Zentmayer, 

 and are now no longer uncommon. Those of 

 Zeiss are upon a formula computed by Prof. 

 Abbe. 



Most makers correct their non-adjustable 

 objectives for a certain fixed thickness of cover 

 and length of tube. In the Zeiss non-adjust- 

 able lenses the thickness of best correction is 



