PHOTO-GALVANOGRAPILY, a Illet11001 of producing from a photograph an electrotype copper-plate in a state fit for printing. It was invented by Mr. Paul Pretsch of Vienna, and is dependent on the property which unaltered gelatine possesses of swellingup in i water. In order that a plate should be fit for engraving, it is of course essential that it should have on its surface elevations and depressions corresponding to the lights and shadows of the picture. Accordingly, the first operation consists in coating a glass with a solution of gelatine and bichromate of potash, and when this is dry, exposing the same to light under a negative. In accordance with the above-named property of gelatine, it will be found, on applying water to the film, that the portion enacted on will swell up. while those parts upon which the actinic rays have exercised their full influence will remain unchanged by the water. From the image thus obtained a gutta-percha mold is prepared, and its surface made conducting by means of levigated graphite or bronze powder. Copper is then deposited thereon by the electrotype process, and the plate thus produced is printed from in the ordinary way.
is a process, invented by Mr. Fox Talbot, for etehing a photograph into a steel plate. It consists in coating the plate with a mixture of bichro mate of potash and gelatine, and exposing under a negative. The effect of this treat ment is to render the gelatine insoluble, just in proportion to the intensity of the light's action, after which a solution of perchloride of iron, of a certain definite strength, is poured over the film, which solution penetrates those parts unacted on by light, reaching the steel plate, and biting itself in, but is repelled by that portioh of time gelatine ren dered insoluble; the plate being thus protected from the action of the solvent. Because a dilute solution of perchloride of iron soaks into a film of gelatine more readily than a strong solution, it is very important that the etching fluid should possess that amount of dilution which has been found by practice to yield the best results.