396 
SUMMARY OP CURRENT RESEARCHES RELATING TO 
to bo impossible to prepare practically useful kinds of glass with 
similar properties to those of fluor-spar. 
This result has served to fix the author’s attention on the use of the 
natural mineral for purposes of practical optics, and more especially for 
Microscope objectives ; for previous experiments in the year 1881 had 
already shown that fluorite, in spite of its less hardness, is susceptible 
of being shaped like glass, although with some difficulty. 
By using clear crystals and cleavage fragments, such as were then 
easily obtainable from mineral dealers, in the year 1884 the optical 
factory of Carl Zeiss in Jena first constructed, under the author’s 
direction, Microscope objectives of different kinds, in which perfect 
correction of the sj)herical and chromatic aberration was effected by the 
use of lenses — one to three in each system — of fluorite instead of crown 
glass. With the introduction of the new Microscope objective, the 
“ Apochromatic,” the mineral has come into regular use in Jena, and 
has been further extended by other opticians in their imitation of the 
Zeiss construction. The calculations and technical details of these con- 
structions have been rendered much easier by the introduction of 
fluor spar in partial replacement of crown glass. Without its aid those 
lens-systems, for the same requirements in the construction, would have 
been still more complicated in composition, and more difficult in manu- 
facture than they are at j>resent. 
In view of this use of fluorite for the Microscope now generally 
admitted, and considering the advantages which it ofiers for many other 
purposes of practical optics, it will be of interest to discuss the deter- 
mining condition of its use, viz. the possibility of procuring this material 
in sufficient quantity and quality. 
The inquiries which the author set on foot many years ago have 
hitherto led to no satisfactory result. Fluor-spar belongs, it is true, to 
the widely distributed minerals, and is found in very many places in 
transparent crystals. Most varieties, however, apart from the rarity of 
large clear pieces, are quite worthless for optical jmrposes. This is due 
to the fact that they show double refraction in a marked degree and 
owing to disturbances of the regular crystal growth. Until some years 
ago, tolerably large pieces, which were water-clear and in parts quite 
pure, could be obtained from mineral dealers. These w ere attributed to 
different, though principally Swiss, localities, and it eeemed reasonable 
to suppose that this serviceable vnriety, free from double refraction, 
would be of quite general occurrence and consequently not difficult 
to procure. More exact inquiries, however, soon proved that all such 
specimens of fluorite, met with amongst dealers or in collections, are 
referable to one and the same locality in the Schwarzhornstock in 
the Bernese Oberland, and in fact, to a single find accidentally made 
there almost sixty years ago. 
According to the communications of Herr E. v. Fellenberg, 
of Bern, and to information which the author obtained later at the 
place itself, a hole, out of which was obtained considerably more 
than 100 cwt. of large water-clear crystals and cleavage pieces of fluor- 
si>ar, was discovered above the Alp Oltscheren by Alpine shepherds 
from Brienzwyler, near Brienz, in the year 1832. This material was 
distributed amongst dealers in minerals in all directions, and after 
