ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
553 
than any possible temporary advantage to be secured in such a peculiar 
way.” 
Dr. S. Czapski,* in reference to Mr. Wright’s letter, writes : — 
“Having read Mr. Lewis Wright’s extraordinary letter (33427) in No. 
1414 of the £ English Mechanic ’ on the subject of the employment of 
fluor spar in apochromatic lenses and the substitution for it of artificial 
opal obtained by Mr. Brun’s method (vide c Archives des Sciences Phy- 
siques et Naturelles,’ t. xxv. No. 6), I will attempt to satisfy his curiosity 
by the following statements. 
As regards fluor spar, Mr. Lewis Wright is labouring under a great 
delusion in assuming that before the use of fluorite was allowed to 
become public, all the known available material had been secured by the 
firm of Zeiss at Jena. The contrary may be said with more truth. The 
firm of Zeiss possessed but a very scanty supply at a time when, even 
previous to Mr. Koristka’s groundless attacks in the ‘ Journal de Micro- 
graphie,’ the fact that fluor spar was being used in the apochromatic 
lenses had been published three times in consequence of information 
supplied by the firm of Zeiss. 
The latter were completely prepared to produce their future apochro- 
matic lenses without having recourse to fluor spar, which by no means 
constitutes the condition sine qua non for the production of apochromatic 
objectives, excepting, of course, in the case of such opticians who can 
only produce them by slavishly copying existing systems. As, however, 
the firm became eventually possessed of a considerable quantity of clear 
material the employment of fluorite in their apochromatic lenses was 
continued. 
But as regards the artificial opal of Mr. Brun, it is, in the first place, 
quite erroneous to treat it as a substitute for fluor spar, for though its 
refractive index is comparable to that of fluor spar (for fluor spar, 
fx d = 1 • 434 ; for opal, fx d = 1 * 450), yet the relative dispersive powers 
are markedly different — viz. 
fx D — fx o 
95-4 f01 ' flu ° r Spar ’ and 67-2 
fXD — 1 
for opal. With the latter the dispersion is, therefore, even greater 
than that attainable with phosphate glasses. For this reason the 
firm of Zeiss wrote about eighteen months ago to Mr. Brun : — 
* That they had abandoned their original hope of a glass to be pro- 
duced by Mr. Brun of exceptionally low relative dispersion, and that 
there remained, therefore, only an exceptionally low refractive index. 
It became now a question of determining in what department of practical 
optics a substance possessing these properties might yield particular 
results, which were not obtainable with other glasses. For as in any 
case the production of your glass will involve greater difficulties, and 
probably also higher costs, than is the case with silicate glasses (which 
also admit of the value of ^ ? being reduced to about 65), and as the 
A [x 
production of large pieces (e. g. for telescopes and large photographic 
objectives) is very likely to cause considerable — perhaps insurmountable 
— difficulties, it would become necessary to determine those cases in 
which its characteristic property, the very low refractive index, con- 
Englisli Mechanic, lv. (1892) p. 287. 
