ZOOLOGY AND BOTANY. MICEOSCOPY. ETC. 319 



gas-furnaces, a gas engine for driving blowers, &c., and with the aid 

 of two assistants for the chemical and the optical part of the work, 

 and of several workmen, the experimental research was continued 

 there for two years. 



The general direction of the work was based on the principles 

 indicated in the Eeport of 1878, and in the paper in this Journal 

 before mentioned. According to these principles, there were two 

 distinct objects: — (1) To obtain a greater variety of the optical 

 properties of the glass in regard to the relation of the refractive to 

 the dispersive power. The existing kinds of optical glass constituted 

 nearly a line, i.e. the dispersion increasing always with the refraction, 

 with very slight deviations only. The object was to combine glasses 

 which, if arranged according to n and A w, would not be confined to a 

 linear series, but would embrace an area of a certain breadth, one 

 value of n admitting various values of A w, and vice versa, as far as 

 possible. 



(2) The second problem was : — To procure kinds of glass of 

 different relative dispersions, in which the dispersions should be 

 proportional, as near as possible, in different parts of the spectrum 

 (the problem of " secondary chromatism "). 



In regard to the general research, Prof. Abbe and Dr. Schott had 

 a predecessor in the late Eev. 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 published about them is very 

 fragmentary 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 account of the glass-samples produced by the Eev. 

 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 100 

 grammes — with a remarkable degree of homogeneity, admitting of an 

 exact 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 effect, 

 by measurement. 



The fusions were obtained by means of gas-furnaces, and with 

 crucibles of very different 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 would have been detrimental to the progress of the work 



