oe; 2S pitt. 
at .- 
116 Tue Microscope. 
two assistants for the chemical and the optical part of the work, 
and of several workmen, the experimental research was contin- 
ued there for two years. 
The general direction of the work was based on the princi- 
ples indicated in the Report 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 @ 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 4n, would not be confined to a linear series, but would . 
embrace an area of a certain breadth, one value of 2 admitting 
various values of 4n, and vice versd, 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 spec- 
trum (the problem of “secondary chromatism ”’). 
In regard to the general research, Prof. Abbe and Dr. Schott 
had a predecessor in the late Rev. W. Harcout, who worked at 
the subject in conjunction with Prof. G.G. Stokes. They could 
not, however, use his results, as al] 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 obser- 
vations being only a confirmation of Prof. Stokes’s account of 
glass-samples produced by the Rev. W. Harcourt (though in 
other essential points the results do no¢ confirm the statements 
of Prof. Stokes). 
Dr. Schott had succeeded, after the first months of his melt- 
ing at Witten, in obtaining fusions of very small quantities— 
down to 100 grammes—with a remarkable degree of homogene- 
ity, admitting of an exact measurement of the refraction and 
dispersion by means of spectrometric observation. This was 
the very best basis of advance, because it allowed of a contin- 
uous and strict co-operation of the chemical and optical research. 
