1886.] 



MICROSCOPICAL JOURNAL. 



91 



' In regard to the general research, 

 Prof. Abbe and Dr. Schott had a 

 predecessor in the late Rev. W. 

 Harcourt, who worked at the subject 

 in conjunction with Prof. G. G. 

 Stokes. They could not, however, 

 use his results, as all that was pub- 

 lished about them is very fragment- 

 ary and very indefinite, and they 

 were obliged to begin quite anew. 

 Nevertheless, one important fact was 

 brought to a practical result, viz : 

 the very peculiar property of boracic 

 acid in regard to the second problem, 

 the new observations being only a 

 confirmation of Prof. Stokes's ac- 

 count of the glass-samples produced 

 by the Rev. W. Harcourt (though in 

 other essential points the results do 

 not confirm the statements of Prof. 

 Stokes). 



' Dr. Schott had succeeded, after 

 the first months of his melting at 

 Witten, in obtaining fusions of very 

 small quantities — down to loo 

 grammes — with a remarkable degree 

 of homogenity, admitting of an ex- 

 act measurement of the refraction and 

 dispersion by means of spectrometric 

 observation. This was the very 

 basis of advance, because it allowed of 

 a continuous and strict co-operation 

 of the chemical and optical research. 

 Every change of chemical composi- 

 tion could be immediately controlled, 

 in regard to the optical efiect, by 

 measvu^ement. 



' The fusions were obtained by 

 means of gas-furnaces, and with cru- 

 cibles of very dift'erent kinds — a great 

 number with platinum crucibles and 

 tools — in quantities of from 50 

 grammes to 12 kilos, according to the 

 particular object, nearly all chemical 

 elements being submitted to trial ; 

 there is even glass containing 10 or 

 20 per cent, of mercury. 



' A large number of analyses had 

 been executed by the assistants up to 

 the end of 1883, and more than 600 

 prisms were ground and measured 

 by the spectrometer. Since then this 

 figure has reached 1000. As it wovild 

 haVe been detrimental to the progress 



of the work to depend on the weather, 

 the spectrometer measurements were 

 always made by means of the five 

 bright lines, Ka, H^/, Na, H/5, H;', 

 after the methods described in Prof. 

 Abbe's paper, "• Neue Apparate," 

 &c. 



' There were innumerable dithcul- 

 ties to be overcome in order to obtain 

 compositions which should not only 

 show the optical properties desired, 

 l^ut at the same time fulfil so many 

 other requirements for optical glass ; 

 and many repeated trials were neces- 

 sary for one and the same subject 

 before a satisfactory result could be 

 obtained. It is due to the ingenuity 

 and energy of Dr. Schott that these 

 obstacles were overcome. 



' Towards the end of 1SS3, Prof. 

 Abbe and Dr. Schott had exhausted 

 the programme, as far as appeared 

 possible in a laboratory-research, and 

 were about to close the aflair and 

 publish the results, as showing the 

 possibilities of a series of new kinds 

 of optical glass, and thereby giving 

 an impulse, as was hoped, to its man- 

 ufacture. At this period, however, 

 several distinguished asti^onomers 

 and phvsicists, who had taken notice 

 of these researches, encouraged them 

 to go one step further, and to imder- 

 take the practical utilization of the 

 results in the way of manufacture. 

 Through the aid of these gentlemen 

 a subsidy was obtained from the Prus- 

 sian Government (though Jena is not 

 in Prussia) to continvie the experi- 

 ments, so as to establish a manufac- 

 ture of optical glass, which did not 

 exist in Germany. Messrs. Zeiss, 

 who had already furthered the work 

 since the beginning in the most lib- 

 eral manner, by putting all the per- 

 sonal and technical resources of their 

 establishment at Prof. Abbe and Dr. 

 Schott's disposal, united with them, 

 and in the beginning of 18S4 glass- 

 works were set up, with a large fur- 

 nance and machinery. The Prussian 

 Government's subsidy was 3000/. , and 

 given under conditions as liberal as 

 any government has ever granted 



