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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 liavc been 

 dissolved out Avith water, showing skeleton traces in the 

 gelatine left behind. 



