398 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
doubtless, if be will visit any of the Derbyshire spar or lead mines — for 
the mineral is a constant companion of lead veins — be will find crystals 
of it quite fit for optical work in overwhelming quantities. Perfectly 
colourless crystals are certainly not very common, neither are they very 
rare ; but, at the same time, are they necessary ? The most common 
colour the mineral assumes is a pale green, evident enough in large 
crystals ; but in laminm thin as the lenses of an objective, scarcely, if at 
all, perceptible. 
‘ Prismatique’s ’ experience of the deterioration of the new glasses 
from atmospheric influences is valuable. It is, at the same time, just 
what a chemist would expect from the composition of some of them, that 
is, if they are honestly named. It must not be forgotten that more than 
half a century ago our own Faraday in England, and Amici, in Italy, 
produced new glasses for optical work which possessed valuable pro- 
i:>erties, and offered great advantages, optically, over those in common 
use. Ross, in London, and Chevalier, in Paris, worked Microscope 
objectives from the new glass, and their performance was said to be a 
great advance upon that of lenses made from the ordinary material. It 
was, however, found that they deteriorated so rapidly that their manufac- 
ture was given up. 
I have now in my possession a 1/10 in. made by Chevalier. The 
outer lenses of the combination, viewed by reflected light, are a bright 
steel-blue in colour, much like the screw-heads in a watch movement. 
The performance must have been phenomenal at the date of its produc- 
tion, for it will even now “ dot ” angulatum ; but the field is filled with 
fog produced by the action of the decomposed surfaces upon the light, 
analogous in effect, but less in degree, to that produced by a very finely 
ground but unpolished lens. With this experience behind us, precisely 
equivalent to that of ‘ Prismatique ’ with the new glasses, it behoves one 
to pause before rushing to the conclusion that the optical millennium 
is here.” 
Mr. Lewis Wright writes * on the same subject in reply to some 
strictures by ‘ Prismatique ’ on the Jena glass and German opticians. 
“ Apart from fluorite altogether, great improvement has been made by 
English and other opticians with the new glass alone; and German 
micro-objectives are now reaching this country superior to any made 
here at double the price. I speak from personal trials, of which I may 
perhaps say a few words another time. Zeiss undoubtedly used at first 
glass which would not bear exposure to the air ; but these things were 
gradually discovered and remedied, though it is too soon yet to say if 
even present lenses will stand permanently. I believe I was, myself, 
the very first to utter a word of caution in these columns on that very 
point, though a glass may be useful in the middle of a triplet which will 
not stand atmospheric exposure.” 
Jena Glass.l — Mr. A. Caplatzi thinks it will be of interest to give 
the list published in 1888, which brings the variety of glasses up to 63. 
The first column contains the number, the second the factory number, 
the third the description, the fourth the refractive index for D, the fifth 
the medium dispersion C to F, and the sixth the specific gravity. At a 
Engl. Mecli., li. (1890) p. 222. 
t T. c., p. 117. 
