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PHOTOGRAPHIC IMPORTANCE OF CHROMIUM COMPOUNDS 85
printer's ink penetrates into the incised parts, which ought to remain white, while the washing of the exposed gelatine films with hot water easily destroys the finer parts of the picture, and this detracts materially from the value of the copies. Moreover, difficulties arise in preparing the electrotype plates ; the film swells up and loses its form. In short, the affair is not so simple as it appears ; little difficulties exist, and these occasion errors which the unprofessional hardly observe, but which considerably diminish the effectiveness of the picture.
At an early period it was found that these processes offered a special difficulty, viz., the reproduction of the transitions from light to shade — the half-tones. These were so very imperfect that the representation of natural objects — portraits and landscapes — was speedily given up, and people confined themselves to the reproduction of drawings, maps, and the like, on an enlarged or diminished scale, and thereby to producing stereotype plates for copper engraving and printing. This application is of no little importance, for it prepares, by the help of light, a metal plate for printing in as many hours as an engraver requires days, and at far less cost.
Gelatine Relief. — We have mentioned above the properties of gelatine, and remarked that it has the power of absorbing cold water and thus swelling up. This property is lost if the gelatine is saturated with chromate of potash and exposed to the light. If this exposure is made under a negative, all the places situated under the transparent parts lose this property, while the other places not affected by the light retain it. Accordingly, if the exposed film be thrown into water, the places which are not affected by light swell, whilst those affected by light are not altered. The result is a true relief, — the lights are in high relief, the shadows in low relief, — and this is so strong that it can be cast in plaster of Paris. For this purpose the relief is dried with blotting-paper, rubbed with oil, and then a paste of