Tue MIcRoscoPE. ph he 4 
Every change of chemical composition 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 chem- 
ical elements being submitted to trial; there is even glass con- 
taining 10 or 20 per cent. of mercury. 
A large number of analyses had been executed by the as- 
sistants up to the end of 1883, and more than 600 prisms were 
ground and measured by the spectrometer. Since then this 
figure has reached 1,000. As it would have been detrimental to 
the progress of the work to depend on the weather, the spectro- 
meter measurements were always made by means of the five 
bright lines, Ka, Ha, Na, Hf, Hy, after the methods described in 
Prof. Abbe’s paper, ‘Neue Apparate,’ ete. 
There were innumerable difficulties to be overcome in order 
to obtain compositions which should not only show the optical 
properties desired, but at the same time fulfil so many other re- 
quirements for optical glass; and many repeated trials were 
necessary for one and the same subject before a satisfactory re- 
sult could be obtained. It is due to the ingenuity and energy of 
Dr. Schott that these obstacles were overcome. 
Towards the end of 1883, Prof. Abbe and Dr. Schott had ex- 
hausted the programme, as far as appeared possible in a labor- 
atory-research, and were about to close the affair, and publish 
the results, as showing the possibility of a series of new kinds 
of optical glass, and thereby giving an impulse, as was hoped, 
to its manufacture. At this period, however, several distin- 
guished astronomers and physicists who had taken notice of 
these researches, encouraged them to go one step further, and 
to undertake the practical utilization of the results in the way 
of manufacture. Through the aid of these gentlemen a subsidy 
was obtained from the Prussian Government (though Jena is 
not in Prussia) to continue the experiments, so as to establish a 
manufacture of optical glass, which did not exist in Germany. 
Messrs. Zeiss, who had already furthered the work, since the 
beginning, in the most liberal manner by putting all the personal 
and technical resources of their establishment at Prof. Abbe and 
