103 



NATURE 



[March 30, 19 16 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



Optical Glass ; an Historical Note. 



The subject of optical glass is, at the present time, 

 one of such paramount importance that no apology 

 is needed for introducing it to the attention of your 

 readers. As is well known, the Rev. Vernon Harcourt 

 and Sir George Gabriel Stokes, in the earlier half of 

 last centur}', laboured together for more than twenty- 

 five years with the object of adding to our stock, new 

 varieties of optical glass, but without success. Their 

 labours, however, were afterwards continued by Prof. 

 Abbe and Dr. Schott, of Jena, who, in the course of 

 some five years, were completely successful. As the 

 result of a critical examination of the work of the 

 English workers. Dr. Czapski — then the head of the 

 firm of Carl Zeiss, of Jena — came to the conclusion 

 that Harcourt and Stokes had failed simply because 

 they had not at their disposal the services of a sym- 

 pathetic and competent glass-maker. 



I have quite recently, by the courtesy of a friend, 

 enjoyed the privilege of reading a number of letters, I 

 believe as yet unpublished, written by Prof. Abbe, 

 during the period of his work on optical glass, to a 

 well-known English microscopist, now dead. One of 

 these letters, dated October 9, 1881, is very interesting 

 because it sets out very clearly the high-water mark in 

 optical construction attained by optical glasses com- 

 mercially obtainable before the Jena glasses were pro- 

 duced. The relevant part of this letter reads as fol- 

 lows : — 



"The Crown and Flint which is applied now by 

 Zeiss — -for objectives, prisms, etc.- — is within the limits 

 of 1-5017 and 1-8017 refractive index for the D-line. 

 The dispersion of the former is 0-00798, and of the 

 latter 0-03287, measured for the interval between lines 

 C and F. The density of the said Crown is approxi- 

 mately 2-40, and of the said heavy flint 5-1. The 

 Crown above is not the ordinary Crown, which yields 

 «D= I"5i5 — i"520 and ?Zk-;?c = 0-00850 - 0-00900 ; it 

 is a special glass of Fell (of Paris). The Flint named 

 above — also from Fell — is not perfectly white, but the 

 colour (yellowish) is not very perceptible in smaller 

 pieces (lenses or prisms). It may be usefully applied 

 for many purposes, though it leaves a rather great resi- 

 dual of secondary chromatism. 



"Fell has made still more refractive Flint, approach- 

 ing 1-9 in index. But this is strongly coloured and not 

 fit for use in my opinion. The common Flint, which is 

 applied for telescope-objectives, has N,, between r6o 

 and i"63, and Np- No between 00165 and o-oi8o. The 

 strongest Flint, wkich is made by Chance Brothers, of 

 Birmingham (i.e. 'double-extra-dense' Flint), has 

 Nd= 171 - 172 and Np— Nc between o"o239 and 0*0241. 



" All taken together, w^e have eighteen difi"erent kinds 

 of Crown and Flint in constant use at Dr. Zeiss 's 

 workshop." 



It is interesting to note that at the time referred to 

 in the above letter Zeiss was entirely dependent upon 

 Chance Brothers, of Birmingham, and Fell, of Paris, 

 for his supplies of optical glass. 



The research work commenced by Abbe and Schott 

 in 1881 on a laboratory scale was so far successful 

 that Prof. Abbe, writing in a second letter on 

 February 21, 1883, says : — 



" Regarding the glass experiments, of which I have 



told you a year ago, 1 may say, that they have had a. 

 very satisfactory progress, as well in regard to the 

 purely scientific aims, for which the research had 

 been undertaken, as in regard to the practical results 

 which are obtained. We are now satisfied that the 

 utilisation of these results for the fabrication of optical 

 glass will be the basis of a good progress of practical 

 optics in several respects. The question is now only 

 how to introduce the results oi the experimental re- 

 search into the fabrication ; for all that can be done 

 in the laboratory is settled now, or nearly settled. 

 For that other aim I have had already, during several 

 months, long and troublesome negotiations in order 

 to obtain for my fellow-labourtr that assistance which 

 could enable him to undertake the practical applica- 

 tion of the long research. Even now, however, it is 

 not yet settled that this will be possible — at least in 

 the manner as it has been planned until now, and 

 within a moderate time. But at all events, the quick 

 utilisation of the research in favour of microscopic 

 optics will not be questionable; we have obtained 

 already, or will obtain within the next time, by mere 

 laboratory operations, sufficient quantities of the new- 

 glasses, which are of interest for the microscope, for 

 enabling Zeiss to begin with the practical applica- 

 tion in this year (which notice, however, I request you 

 to consider as a private one at present, because it 

 would not be agreeable, to have this matter spoken 

 of long before it is a matter of fact)." 



This letter is very interesting, because it shows 

 that at the time in question, so far as the compara- 

 tively small quantities of special glasses required for 

 the production of microscope objectives was con- 

 cerned, the laboratory output was sufficient to enab'h 

 the work to be done. This fact at once points to th( 

 possibility of meeting the demand at the present tim; 

 for very special glasses required in small quantities 

 only, as, for example, the production of microscop;- 

 objectives by laboratory rather than by factory 

 methods. 



The production of glass on 

 was commenced at Jena in i8{ 

 a successful conclusion in 188 

 logue of the Jena glasses was issued. 



The third letter written by Prof. Abbe is dated 

 March 4, 1886, and wafe accompanied by one of the 

 first — if not the first — homogeneous immersion apo- 

 chromatic microscope objectives made. The letter 

 reads as follows : — 



"This is a homog. immersion of 1-40 apert. and 

 30 mm. focal-length, constructed by means of new 

 kinds of optical glass which have been produced on 

 the base of a systematical research into the optical 

 qualities of the various elements admitting of vitrifica- 

 tion. This research has been conducted through about 

 three years in the way of laboratory work, chemical 

 and optical, by myself and a fellow-labourer of ti 

 chemical and technical line (Dr. Schott) with the cor 

 tinuous assistance of two younger scholars, chemis^ 

 and phj^sicists ; and has afterwards — nearlv two yeai 

 ago — induced the foundation — at Jena — of a technics 

 establishment for the regular fabrication of all kindsl 

 of optical glass for general use. This glass-manufac-J 

 tory (which has been set up in 1884 by Dr. Schott.J 

 Messrs. Zeiss, and myself, with the aid of a subsidjj 

 of the Prussian Government) has taken up, and conT 

 tinued, the former experiments on the scale of fabricaf 

 tory work, in order to make the results available foif 

 the various branches of practical optics. This is goinji 

 on still — some tasks being settled (the production of thi;^ 

 silicious glasses, which is in a regular fabrication sinoi 

 last summertime), other tasks being brought near tj 

 the aim. In the meanwhile, I have gone to work wit'j 

 theoretical research and computation; in order to fin 

 the proper formulas for the utilisation of the ne\ 



manufacturing scale 

 and was brought to 

 when the first cata- 



