MICROSCOPY. 



509 



to say that objectives anywhere from $ to T V 

 inch, if not lower, can now be obtained which 

 will show, as well as has ever been done, any- 

 thing that has yet been seen by the micro- 

 scope. 



In the " homogeneous " immersion objective, 

 the front lens, by the interposition of a liquid 

 having the same refractive index as itself, 

 is practically extended beyond the balsam- 

 mounted object, or close to the dry-mounted 

 one; by which means all the advantages of 

 the immersion system, which are only approxi- 

 mately attained in water immersion, are fully 

 secured. The homogeneous arrangement, 

 which is now adopted in all work of the high- 

 est grade, was partially anticipated by Amici, 

 "Wenham, and Tolles, and closely approached 

 by the glycerine immersions of Gundlach, 

 Spencer, and Tolles, and was finally realized by 

 Mr. Zeiss, of Jena, in 1878, at the instigation 

 of Mr. J. "W. Stephenson, and under the advice 

 of Prof. Abbe (" J. K. M. 8.," 1878, p. 61 ; 

 1879, p. 256). As the varying thickness of 

 immersion fluid exactly compensates, theoreti- 

 cally, for the inverse variation in cover glass, 

 and as the selection of the point of best cor- 

 rection presents great difficulties and uncer- 

 tainties in the case of histological specimens, 

 Zeiss and some others first made these systems 

 non-adjustable, according to the original sug- 

 gestion (above), and with such distinguished 

 approval as that of Dr. Carpenter ("Encyc. 

 Brit.," xvi, 1883, p. 265), Prof. Abbe, Dr. 

 Dippell, and Mr. Ingpen, all of whom favored 

 the use of even adjustable systems at a single, 

 carefully selected, andfixed average adjustment. 

 To compensate, howevef, for differences in 

 oculars, tube-length, refractive index of covers, 

 or depth or refractive index of the mounting 

 medium, to all of which these objectives are 

 very sensitive on account of their large aper- 

 ture, it is doubtless preferable, at least for skill- 

 ful manipulators, to have an adjustment, as 

 early advocated by Drs. H. Van Heurck and 

 George E. Blackham, and by Messrs. Spencer, 

 Mayall, etc., and this is now added by most 

 makers. The adjustment is likewise needed 

 to enable the same objectives to be used, on 

 occasion, for water or glycerine immersion ; the 

 latter at least being adequate for much histo- 

 logical work, and being therefore often sub- 

 stituted for the more intractable and trouble- 

 some though more nearly homogeneous oils, 

 resins, gums, or dense saline solutions.* As 

 the adjustment is not indispensable, such lenses 

 may be set in very short mountings for use 

 with binoculars which, like the Wenham and 

 Stephenson, require the objective to be near 

 the prism ; a Powell and Lealand ^, thus 

 mounted, having given both fields well lighted 

 with the Wenham prism. 



In the theory of the objective but little prog- 

 ress was made after the Tolles- Wenham con- 



* See Dr. Van Heurck's suggestions on homogeneous fluids 

 in "Bull. Soc. Belg. de Mic.," 1881, p. 11. and in "A. M. M. 

 J.," 1S82, p. 26. 



troversy established the practicability of util- 

 izing in immersion lenses an interior angle of 

 light in excess of that corresponding to 180 in 

 air, until Prof. Abbe, the distinguished and 

 versatile theoretical microscopist of Jena, gave 

 us the doctrine of resolution by diffraction and 

 a rational theory of aperture and its relations 

 to other qualities of objectives. Prof. Abbe, 

 applying to the objective the researches of 

 Helmholtz and himself on the wave theory of 

 light, showed that dioptric resolution by re- 

 fraction, analogous to ordinary vision, is by 

 nature limited to details of structure not less 

 than 2^00 inch apart ; and that details finer 

 than this, in the microscopic field, are filled in 

 by a coincident image formed by the recombi- 

 nation by the objective of diffraction bands 

 formed by interference of light at the edges of 

 the object or of its details. Lenses of insuffi- 

 cient aperture to collect all the spectra from 

 points or lines of a given degree of closeness 

 must, therefore, give a false view of these de- 

 tails, or no view at all, and the power of reso- 

 lution for light- waves of a given length has a 

 definite ratio to the aperture of the objective 

 (for Fraunhofer line E = a x 96,400), a limit 

 not yet quite attained in practice, and not ca- 

 pable of being exceeded by any means yet 

 known and understood.* 



The angular aperture, by which objectives 

 had been hitherto characterized, gave no sat- 

 isfactory measure of their working qualities, 

 even when dry, and wholly failed to account 

 for the gain in brilliancy of view and in power 

 of resolution by immersion systems. Prof. 

 Abbe also demonstrated that the practical ap- 

 erture, or capacity for receiving image-form- 

 ing rays, is not the angular aperture, but is the 

 ratio between the focal length and the utilized 

 diameter of the system (sine of the semiangle 

 of aperture = sin. u), this varying still further 

 with the index of refraction () of the im- 

 mersion liquid. Thus the numerical aperture 

 (a = n sin. u) of a dry lens of the theoretical 

 limit of 180 will be I'OO, of a water immer- 

 sion lens of 180 it will be 1'33, and of a ho- 

 mogeneous immersion of 180, 1*52 ; these 

 numbers representing the relative resolving 

 powers of the several combinations, while the 

 illuminating powers vary as the squares of the 

 apertures, and the penetrating powers, or depth 

 of fields, vary in the inverse proportion. From 

 similar comparisons it follows that a dry lens 

 of 180 is only equivalent in aperture (1*00) to 

 a water immersion of 97 31', and to a homo- 

 geneous immersion of 82 17'; all these having 

 the same illuminating and penetrating power 



* The largest apertures now commonly made are 1' 30 to 

 1-40; and the largest claimed for any objective yet made is 

 about 1'47, whose limit should be about 141,000 lines to the 

 inch. The closest lines yet seen, in the writer's judgment, 

 are 120,000, which seem to have been fairly seen on both 

 Fasoldt's and Kogers's plates. The single claim to have seen 

 152,000, more than the limit for homogeneous immersion ob- 

 jectives in medium of 1 '52, is unsupported by such proof or 

 corroboration as to justify its acceptance. The 120,000 lines 

 are theoretically within the limit of water-immersion systems, 

 which have been carried up to a (claimed) na of 1'30. 



