January 30, 19 13] 



NATURE 



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render it almost impracticable, Czapski remarks in 

 thu Zcitschrift fiir wistieiisch. Mikroskopie, vol. viii., 

 1S91, p. 149, as follows: — Organic preparations re- 

 quire from their very nature to be embedded in media 

 which in thu majority of cases have a much lower 

 refractive index than the immersion fluid for which 

 the objective has been computed. This excludes all 

 these preparations from observation with the mone- 

 brome naphthalin lens, since one of the principal 

 conditions for the successful use of the lens remains 

 unfulfilled. Even tlie difficulties which attend the 

 use of the objective and its high price could never 

 have been regarded as a sufficient reason for dis- 

 pensing with its services if any considerable range ot 

 objects existed which could bear being embedded in 

 the media having the requisite optical properties, and 

 in which the capacity of the lens could be turned to 

 full account. As it is, the objective is only known 

 by the photograplas taken by Van Heurck, and, as 

 a matter of fact, the lens has long ceased to be 

 manufactured. 



There is yet anothei way of enhancing the working 

 capacity of a lens, as will be seen from the formulae of 

 Helmholtz. The wave length of light may be reduced 

 by working with white light having a wave length of 

 o'oooss mm., or blue light having a wave length of 

 o'ooo43 mm., or ultra-violet light having a wave 

 length of o'ooo28 mm. Blue light reduces by about 

 1/4-1/ 5 of its original value, whilst ultra-violet light 

 reduces it to about one-half. A few years ago Dr. 

 A. Koehler investigated successfully what might be 

 accomplished in this direction by the use of light of 

 extremelv short wave lengths (Zeitschr. /. laiss. Mikr., 

 vol xxi., 1904, p. 129 et seq.). An elaborate new 

 apparatus was required to obtain tangible results 

 from the application of very short waves. Even the 

 best glasses that will transmit ultra-violet light did 

 not suffice for the purposes of this investigation, and 

 the only materials which transmitted ultra-violet light 

 of sufficient intensity were fused quartz and fluor- 

 spar. The condenser, the object slide, cover glass, 

 objectives, and eyepieces had all to be made from 

 either of these materials, whilst glycerin served as the 

 immersion fluid. Only a limited number of mounting 

 media were available for use with this apparatus. 

 The lacking intensity of this light rendered it impos- 

 sible to apply it visionally, and recourse was accord- 

 ingly had to the photographic method. The difficul- 

 ties encountered in focussing the object have been 

 overcome by the application of fluorescent light. 



The greatest difficulties were encountered in the 

 construction of the objectives. Owing to the limited 

 choice of materials it was impossible to attempt to 

 make the lens achromatic, and indeed this was 

 scarcely a matter of importance, seeing that the light 

 used is almost monochromatic. On the other hand, 

 the use of simple lenses made it impossible to secure 

 spherical correction with respect to more than a small 

 central aperture. This applies at least to a high 

 power dry lens and to an oil-immersion lens of low-er 

 power. 



There is another circumstance which was found to 

 be a serious drawbaclc, in that the lenses of the 

 objectives had necessarily to be mounted without the 

 usual adjustments by means of w'hich departures in 

 the radii, thickness, distance between the lenses, and 

 irregularities in the homogeneity of the glasses may 

 be allowed for, since it is almost beyond the re- 

 sources of a workshop to apply any direct test to 

 lenses corrected with respect to the ultra-violet light. 

 It will be readilv appreciated that an objective which 

 is made exclusivelv on the strength of data obtained 

 bv calculation without the controlling aid of the 

 optician's art must necessarily be of the nature of 



NO. 2257, VOL. go] 



chance products. In these circumstances one may 

 either dispense with the liighest degree of perfection 

 by accepting an objective as it leaves the optician's 

 hands, or one may from one of a series of several 

 lenses select the one w-hich is best, by direct observa- 

 tion with a fluorescent screen, with a fluorescent eye- 

 piece, or bv photographic tests, but such a proceed- 

 ing- would be inordinately costly. Koehler's investi- 

 gations have the merit of having clearly demonstrated 

 the almost insuperable difficulties with which one has 

 to contend when attempting to apply ultra-violet 

 light. The photographs which have been obtained 

 so far with the aid of ultra-violet light have scarcely 

 furnished any new aspect of the structure of micro- 

 scopic objects. 



Apochromatic and Fltiorite Lenses. 



The labours of Abbe and Schott in the study and 

 production of optical glasses, which were begun in 

 i88i, were to a certain extent completed in 1886. In 

 the technical laboratory established by them under the 

 title of Schott und Genossen, they brought out, in 

 addition to the crown and flint glasses then in use, 

 an extensive series of glasses having markedly im- 

 proved optical properties and of a different chemical 

 composition. By introducing phosphoric acid and 

 boric acid as components of glass smeltings, in addi- 

 tion to silicic acid, they succeeded in producing new 

 crown and flint glasses, the so-called phosphate and 

 borate glasses, in which the rate of change of the 

 dispersion is remarkably proportional, so that it 

 appeared possible by the combination of these glasses 

 partly to eliminate the secondary spectrum, which 

 hitherto it had been impossible to eliminate to any 

 appreciable extent. In the course of time the Schott 

 works introduced an extensive selection of optical 

 glasses, which greatly simplified the computing 

 optician's work. The list includes glasses of similar 

 refringent properties but widely different dispersion, 

 and others again having a similar dispersion but 

 covering a wide range of refractive indices. This 

 was a great advance over the old glasses, in which 

 any increase of dispersion was attended with a rise 

 in the refractive power. 



A valuable feature of the new glasses is that the 

 glass works w-ere able to reproduce very closely any 

 of the types specified in their catalogues, and, in 

 addition, every new pot was examined with the 

 spectrometer and its constants recorded. This relieves 

 the computer of the task of having to determine for 

 himself tlie optical properties of the glasses, and like- 

 wise the optician working on the trial-and-error prin- 

 ciple was enabled more easily to attain his purpose 

 by a judicious variation of the glasses in accordance 

 with their refractive properties and dispersions. These 

 glasses were used for the first time in apochromatic 

 objectives as originated by Zeiss. This would indeed 

 have been a stupendous achievement if, as the makers 

 of these lenses maintained at first, their success had 

 been solely due to the use of the new phosphate and 

 borate glasses. Unfortunately, as we shall have 

 occasion to show, it was the introduction of fluorite 

 into the composition of the lenses which was respon- 

 sible for these achievements. 



The new objectives, which were completed in 1886, 

 proved a great advertisement for the glass works 

 of Messrs. Schott und Genossen. Of the use of 

 fluorite, however, not a word was uttered, even in a 

 lecture delivered by Abbe on the subject before a 

 scientific gathering and published in the Transactions 

 of the Jenatsche Geselhchaft fiir Medizin und Natur- 

 ■wissenschaften, under the title, " Ueber neue Mikro- 

 skope," or, as it appeared in a subsequent reprint, 

 " Ueber Vcrbesserung des Mikroskops mit Hilfe neuer 



