To Professor Husnik (to whom collotypists are greatly indebted for so many unselfish publications), we owe the origination of this interesting method of producing “magic” prints. It is founded on the reaction well known to chemists that two substances, each colourless in itself, when mixed, enter into fresh chemical combinations and produce other substances of intense colour. Herschel and Hunt made use of many of these reactions, both to develop the invisible image, and to give to others, already visible a more agreeable colour or tone. Impressions other than those in fatty ink may be obtained from the Collotype plate—as in the process patented by Edwards—by means of colour, soluble in water, to which the addition of a small amount of gum has been made. In such cases it will not be those portions of the layer which have been rendered insoluble by the action of light which will give the impression (as it would if ink were used) but on the contrary, the unexposed parts, or those which remain soluble. On this account the resulting print will be a negative, and to obtain a positive, the printing must take place under a positive. If a Collotype plate so produced be damped with a linen rag which has been moistened with an aqueous solution of a salt readily absorbed by gelatine, and which will produce in combination with a second solution an intensely coloured compound (the use of the ink roller being entirely omitted), the impression on the printing paper will be almost or even entirely invisible until brought in contact with the second solution, when the picture will be immediately developed and rendered visible. For instance, dampen the plate with an aqueous solution of red prussiate of potash, surface dry it, print upon paper, allow to dry thoroughly, and place upon a solution of protosulphate of iron, when the picture will immediately appear of an intense blue colour. Solutions of bichromate of potassium or nitrate of silver will produce brown images; the former developed with extract of logwood, alazarine or sugar of lead, all produce different colours. Not only is this process interesting, applied in the manner already described, but it is of use in calico printing. Care must however be exercised in the selection of a suitable solution for damping the plate, as many salts would tan the gelatine layer, and such are of course utterly unsuited for the purpose. Among the mordants in common use, salts of tin, alum, iron, &c., may be mentioned as exercising a hardening influence upon gelatine, and therefore useless for moistening the layer. There remain, however, many others which produce no damaging effect upon the plate, and at the same time produce useful colours. CHAPTER XV. PHOTO “GLASS” PRINTING.[N] [N] This process has never been practised in this country so far as I am aware.—TRANS.