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coloured lights of the spectrum in succession, instead of 
ordinary light. He finds that details of structure are more 
distinctly brought out by some of the colours than others ; 
the blue and green rays are also very pleasant to work with, 
and easily varied by throwing the required part of the 
spectrum on the mirror below the stage. 
Mr. Sidebotham brought before the notice of the meeting 
Mr. Petschler’s process for producing vegetable forms with 
crystals of bichromate of potash in gelatine, which was 
discovered by him in the preparation of glass plates for 
photographical purposes, and exhibited at the Microscopical 
Soiree given to the British Association at the last meeting. 
Specimens on large glass plates were handed round, which, 
when magnified, aptly represent mosses, ferns, and algae, in 
beautiful ramifications, which vary in many ways, dependent 
upon the strength of the solution, temperature, state of the 
atmosphere, and other causes. Mr. Sidebotham called 
especial attention to the peculiarity of the form of crystallisa- 
tion, and to the fact that an inorganic salt, in contact 
with organic matter, should produce vegetable forms. 
The Secretary then read a Paper by Mr. Petschler, 
describing the plates and the process. 
Glass plates, Nos. 1, 2, and 3, were coated with collodion, 
on the surface of which a hot mixture of gelatine and 
bichromate of potash had been poured, then allowed to cool 
and to dry spontaneously. In a few hours the crystals began 
to form and ramify themselves over the plate. The mixture 
was composed of three parts of gelatine and water twenty 
grains to the ounce, to one part of a saturated solution of 
bichromate of potash. 
Plate No. 4, the same mixture spread hot without 
collodion. On a corner of the plate the crystals have been 
dissolved out with water, showing skeleton traces in the 
gelatine left behind. 
