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PHOTOGRAPHIC IMPORTANCE OF CHROMIUM COMPOUNDS 91
originally to the right in the lower image comes now to the left. That this is so can be easily seen by writing a word with thick ink in large letters on a piece of paper, and laying a piece of blotting-paper on the wet writing. The reversed impression of the writing is seen on removing the blotting paper. In the letter copying-press the same thing takes place, and therefore the letters are copied on very thin paper, that they may be read on the reversed side, because viewed from that side they appear in their first position. The gelatine prints cannot be printed on such thin paper ; therefore, if the reversed position is inconvenient, the picture must be again transferred. This is managed in the following manner : —
The moist gelatine film is placed after exposure upon a smooth zinc plate, to which on drying it becomes very firmly attached. The copy thus glued to zinc is immersed in warm water, the paper becomes detached, and the developed image adheres to the zinc plate. A sheet of white paper is now fastened with glue upon the zinc plate, and allowed to dry. The gelatine picture adheres firmly to this paper, and may, with care, be detached from the zinc plate. The picture thus appears unreversed upon the paper.
The pictures thus obtained are very similar to those of the Woodbury-type ; they surpass the latter, however, in the fineness of their details and the ease with which they may be produced.
The pigment impressions have a great advantage in the fact that they can be produced in any colour ; genuine Indian ink may be used for them, and then perfectly durable pictures are obtained that do not turn yellow or black.
In the same way red, sepia, blue, and so on can be mixed with the gelatine, and thus pictures can be produced in those colours. This circumstance is important when it is wished to reproduce coloured drawings.