6o6 



NATURE 



[January 30, 19 13 



Arten optischer Glaser." Again, not a word was said 

 of fluorite in a paper entitled, " On Improvements of 

 the Microscope with the Aid of New Kinds of Optical 

 Glass," which appeared in the Journal of the Royal 

 Microscopical Society, vi., 1886, p. 20, et seq. ; neither 

 was it mentioned in a publication prepared for French 

 readers, and entitled, " Nouveaux Objectifs et 

 Oculaires pour Microscopes construits avec les verres 

 speciaux de la Verrerie scientifique (Schott et Cie.), 

 par Carl Zeiss, Atelier d'Optique h. lena." 



Four years passed before the true facts of the case 

 were made known by the firm of Zeiss in an article 

 published in the Zeitschrift fiir Instritmentenkunde, 

 1890, p. I, under the heading, " Ueber die Verwend- 

 \ing des Fluorits fiir optische Zwecke." Long before 

 the publication of this paper other firms, having failed 

 to produce lenses equivalent to the apochromatic 

 lenses with the aid of the new glasses only, had 

 realised that the great advances in achromatic cor- 

 rection embodied in the apochromatic lenses were not 

 due in the first instance to the use of the new glasses 

 but rather to that of fluorite. Doubtless Abbe had 

 set himself the task of achieving chromatic correc- 

 tion of a higher order by means of the new glasses 

 only, but he failed in arromplishing any very striking 

 results with the aid of these glasses only, and, more- 

 over, the optically most trustworthy glasses could be 

 used in a very restricted sense only, owing to their 

 lack of resistance to atmospheric influences. 



We will now briefly discuss the conditions under 

 which glasses combined to form crown and flint glass 

 pairs will furnish a means of securing a more or less 

 complete degree of spherical and chromatic correction. 



The older lenses of the achromatic type are com- 

 posed of two doublet lenses for the lower and moder- 

 ately high powers, with one or two front lenses of 

 crown glass added for the high powers. The doublet 

 lens consists of a negative flint glass component and 

 a positive crown glass component cemented thereto. 

 The spherical correction is in the main effected at the 

 surface of contact between the components of the 

 doublets." The magnitude of the difference in the 

 refractive indices of the flint and crown glass com- 

 ponents governs the curvature of the cemented sur- 

 faces, and it should be as high as possible to flatten 

 the surface and to adduce favourable conditions for 

 the correction of the spherical aberration and for 

 securing a high aperture. To eliminate the chromatic 

 aberration the glasses are so chosen that the disper- 

 sion of the highly refracting flint may be considerably 

 greater than that of the less refringent crown glass. 

 .'V good gauge of the dispersive properties of a glass 



is furnished by the formula — f, -^ = i' : nF, nD, nC 



denote the refractive indices with respect to the rays 

 corresponding to the F, D, and C lines of the spec- 

 trum; nF— nC stands for the mean dispersion; 

 i?F— nC ,. .,,,.. 



-pT supplies an expression for the dispersive 



power, whilst its reciprocal value, usually denoted by 

 the letter i', is known as the efliciency of the optical 

 medium. 



In the list of glasses made by Messrs. Schott and 

 Co. the glasses are arranged in a progressive order 

 of ascending values of the efliciencv v. Glasses in 

 which the value of v ranges from 75 to 55 are usually 

 classed as crown glasses, whilst those in which " has 

 a smaller value go by the name of flint glasses. The 

 combination of a positive crown glass lens with a 

 negative flint glass lens affords a means of correcting 

 the chromatic aberration. 



\ higher degree of achromatisation can be attained, 

 i.e. the secondary spectrum may be eliminated and 

 rays niade to meet in a point with respect to more 



NO. 2257, VOL. go] 



than two colours, if glasses are chosen in which the 

 dispersions proceed by proportional steps. The degree 

 to which this requirement is satisfied may be ascer- 

 tained by dividing the difference of the refractive in- 

 dices for tvi'o fixed lines of the spectrum, say F and 

 G', i.e. the so-called partial dispersion, by the differ- 

 ence of the refractive indices for the interval C to F- 

 the so-called mean dispersion. A pair of crown and 



flint glasses, in which the quotients —^^ „ differ 



nV - /;C 



least will be best adapted for the achromatisation 



of an optical combination with respect to a third 



colour. 



Achro?natic Lenses. 



The above table shows in the first section a pair of 

 glasses such as are used to produce an achromatic 

 lens, and it will be seen to what extent the refractive 

 indices of the components nD and the values of i' should 

 differ to effect the requisite spherical and chromatic 

 corrections. It will be seen that the quotients given 

 , , , , , . partial dispersion ,.„ 



in the column headed q, viz. r^ - — ^ > diner 



^ mean dispersion 



by an amount ^9 = 43 to 49 units. With this differ- 

 ence remaining there is still a pronounced secondary 

 spectrum, since the imperfect proportionality in the 

 configuration of the spectra due to the glasses renders 

 it impossible to bring three colours to a point. 



The second section typifies the new glasses which 

 were employed to effect a higher degree of correction 

 in achromatic lenses. In the first place, the differ- 

 ence in the refringent properties of the phosphate 

 crowns and borate flints was not sufficient to obtain 

 such flat lens curvatures as are needed to ensure a 

 large aperture, at least not with a single pair of 

 glasses. The quotiental difference, Ag, is in these 

 glasses brought down to 15-31. This signifies already 

 a very marked advance, and to improve still further 

 upon it one would have to have recourse to denser 

 flint glasses of greater refractive power so as to obtain 

 better conditions for correcting the spherical aberra- 

 tion by flattening the curvative. Even if it had 

 proved possible, by complicating the formula, to evade 

 the presence of pronounced curvatures and to use 

 glasses of a small quotiental difference only, there 

 would still have remained an insurmountable difficulty 

 in that all borate and phosphate glasses are so little 

 permanent as to exclude their use in lenses. An ob- 

 jective containing elements made up of these mate- 

 rials, whilst produced at a greatly increased cost, 

 could not have failed to become useless in a very 

 short time. Those lens-makers who used these glasses 

 before they had had time to realise their peculiarities 

 had to pay dearly for their subsequent experience. ; 



The third section of the table comprises fluorite and 

 a number of glasses with which it may be associated 



