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480 History of Three-Color Photography
screens by softening and crushing the colored elements ; but whether this particular method was ever adopted is not known, but certainly commercially no one has ever reported the occurrence of Autochrome plates without the black filling.
H. Clement15 would replace the starch grains by corpuscles infinitely small, rigorously of the same size, ultratransparent and yet capable of being stained by the absorption of colors when alive, these corpuscles being the spores of champignons, algae, mosses and the like, of which the growth is arrested at the desired moment. E. Gistl16 while recognizing that fine glass particles had been suggested, pointed out that in melting the same to make them adhere to the glass, they would assume a more or less spherical form and would then act as refractive elements, therefore, he proposed to grind and polish them flat, after firing.
The Aktien Gesellschaft f . Anilinfabrikation, usually known as "Agfa," introduced17 a screen-plate that was prepared from gum arabic ; this being colored, broken up and then deposited on glass, and by suitable treatment pressed flat, so that there were no interstices. The margins of the elements frequently overlapped so that there were black outlines. As the gum would be attacked by aqueous solutions, the film was provided with an insulating varnish, which was zapon or celluloid varnish, about 0.02 mm. thick. For further notes on this plate see page 578.
J. Bamber18 proposed to use gelatin as the color carrier, and sheet gelatin was stained up and dried, then immersed in a 15 per cent solution of formaldehyde and dried. This last treatment made it very hard and brittle in water, in which it was then soaked and allowed to expand to its fullest extent; excess water was removed and it was ground up to a fine granular state. Grinding should be effected under 75° C. and the powder allowed to dry, then further grinding carried out. The powder, thus obtained was of various sizes and elutriated in petroleum spirit, about sp. gr. 0.680, or some other spirit that it did not absorb. By this process it was possible to select grains in equal batches of from 1/300 to 1/3000 of an inch. Transparent celluloid was then coated with a solution of celluloid and a resinous gum that remained tacky, and the colored powders, suitably mixed, sifted on, having been previously dried at 100° C. The screen thus formed was exposed in a room under normal conditions, so that the gelatin grains expanded by absorption of moisture from the air, and there would be interlocking of the grains, so that no white light passed, or a warm solution of gelatin might be sprayed on for the same purpose. After expansion the screen was passed through polished pressure rolls, slightly heated ; there was thus obtained a perfectly even surface, which after being varnished with celluloid, might be again passed through the rolls slightly heated and be coated with panchromatic emulsion. Glass plates might be used and be coated with pale gold size to form a tacky surface.
In a subsequent patent19 Bamber proposed to avoid the use of the com