ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 337 



commercially obtainable before the Jena glasses were produced. The 

 relevant part of this letter reads as follows : — 



" The Crown and Flint which is applied now by Zeiss — for objectives, 

 prisms, etc.— is within the Hmits 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 approximately 2-40, and of the said heavy 

 flint 5-1. The Crown above is not the ordinary Crown, which yields 

 /?,. = 1-51.5 - 1-520 and n, - n, = 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 residual of secondary 

 chromatism. 



" Fell has made still more refractive Flint, approaching 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 

 1-60 and 1'63, and N, - N„ between 0-0165 and 0-0180. The 

 strongest Flint, which is made by Chance Brothers, of Birmingham 

 (i.e. 'double-extra-dense' Fhnt), has N,, = 1-71 - 1*72 and N, - N,, 

 between 0-0239 and 0-0241. 



"All taken together, we have eighteen different kinds of Crown 

 and Flint in constant use at Dr. Zeiss's w^orkshop." 



It is interesting to note that at the time referred to in the above 

 letter Zeiss was entirely dependent upon Chance Brothers, of Bir- 

 mingham, and Feil, 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 siTCcessful that Abbe, writing in a second 

 letter on February 21, 1883, says : — 



" Regarding the glass experiments, of which I have told you a year 

 ago, I may say, that they have had a very satisfactory progress, as well 

 in regard to the purely scientific aims, for which tlie research had been 

 undertaken, as in regard to the practical results which are obtained. 

 We are now satisfied that the utilization of these results for the fabrica- 

 tion 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 of the experimental research 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-labourer that 

 assistance which could enable him to undertake the practical appUcation 

 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 utiliza- 

 tion 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 application in this year (which notice, however, 

 I request you to consider as a private one at present, because it would 



