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280 THE ADVANCE OF PHOTOGRAPHY
To develop the image all that is necessary is to remove the unnecessary parts of the bichromated gelatine by the continued application of cold water.
The collotype plate so obtained must next be prepared for the printing-press by first removing from its back any stray gelatine, and then subjecting it to the so-called " etching process/' which must not be confused with the " etching " already dealt with in connection with zinc and copper plates.
In this case the surface of the plate is exposed to the action of a solution made up of one ounce gelatine, 2 ounces water, and 2 drops of ox gall, for about half an hour.
When this solution has been removed by means of a sponge, the plate is placed upon the bed of the press and the film dried with a soft cloth, after which it can be inked up by using leather rollers slowly and with gentle pressure. If the plate has been properly prepared, shadows will first take the ink and then the half-tones, while the high lights should remain almost, if not entirety, free from it. It is usual to make use of a second roller with ink slightly less thick, and rolled with less pressure and more quickly, as this is found to be a very effective manner of removing surplus ink. After placing upon the plate the necessary mask, it is now ready for the printing process.
It can readily be conceived that paper suitable for this process must be of the very best quality and have a smooth surface. The frontispiece shows an illustration prepared by this process.