552 SUMMARY OF CURRENT RESEARCHES RELATING TO 
were used throughout. For the image shown by the immersion lens 
the exposure was about 1J minutes, with correspondingly shorter ex- 
posures for Zeiss’ A and D. 
(6) Miscellaneous. 
Ink for Writing on Glass or Porcelain.* — The ‘Rundschau’ 
(Prague) gives the following: — Dissolve in the water-bath 10 parts 
bleached shellac and 5 parts Venice turpentine in 15 parts oil of 
turpentine. Incorporate in the solution 5 parts of lamp-black. So- 
called diamond ink for writing on glass is a compound of fluoric acid 
and barium ; the latter has no effect, it being simply a white powder to 
give body to the acid. The ink can be used with a rubber hand-stamp, 
and it should be allowed to remain fifteen minutes, when the barium 
will brush off, leaving the design on the glass. 
Spherical Aberration— Apochromatic Objectives, t — Mr. Lewis 
Wright writes as follows : “ Allow me to draw attention to another subject 
which vitally interests English microscopic opticians — viz. the produc- 
tion of apochromatic objectives. Though some of them have managed 
to secure a little supply, others are painfully aware 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 ; and the difficulty of getting 
material experienced by some of our best makers is a formidable obstacle 
to optical improvement, and tends to artificially keep up the prices. 
From an American periodical lent mo by Messrs. Watson and Sons, I 
see that Prof. J. Brun has recorded in the ‘Journal de Micrographie ’ 
the success of M. Albert Brun in producing by chemical synthesis what 
is called ‘ artificial opal,’ and which is stated to have almost exactly the 
optical properties of fluorite, but to be harder, more homogeneous, and 
better suited for optical working. It is also stated that the process 
produces pieces large enough to be conveniently used in the manufacture 
of optical instruments. But this is the point. Prof. Brun apparently 
quotes from ‘ Archives des Sciences Physiques et Naturelles,’ of Geneva, 
June 1891, and further quotes from it, or else states himself, that ‘ the 
house of Carl Zeiss , at Jena , has acquired the right to manufacture and to 
use this artificial opal for optical purposes' 
I think microscopical opticians have a right to ask what this means. 
It may mean no more than that the Jena firm has purchased a right to 
use this newly-available material, which is fair enough ; indeed, I am 
such a baby as regards patent law, that I am not even sure whether or 
not a really international monopoly can be thus bought and sold. But 
if, as on its face appears, it is here meant that the German house has 
also effected a monopoly of this artificial process, or attempted to do so, 
and thus to obstruct the fair play of competition and march of improve- 
ment, the sooner such a peculiarly German method of business is 
generally understood by microscopists the better. I sincerely trust the 
affair is one of only clumsy translation, and that some one authorized to 
speak on behalf of Messrs. Zeiss will be able to dispel any such 
suspicion, which would assuredly do them more harm in the long-run 
* Amer. Mon. Micr. Journ., xiii. (1892) p. 110. 
t English Mechanic, lv. (1892) pp. 220-1. 
