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PHOTOGRAPHIC IMPORTANCE OF CHROMIUM COMPOUNDS 95
reduced to oxide of chromium, which does not affect aniline colours ; on the other hand, the chromic acid remains unchanged under the black strokes, which keep back the light. After the exposure a very faint picture of unchanged yellow chromic acid is seen.
If this faint picture is exposed to the vapour of aniline, a brown colour is produced at the places where the yellow strokes exist, and in this manner the original faint yellow becomes well defined. To expose the prints to aniline vapour they are placed in a covered box with a piece of blotting-paper, moistened with a solution of aniline in benzine. This process produces a positive picture from a positive, and is therefore very valuable in producing faithful copies of drawings. These copies are, of course, reversed, as if seen in a mirror. This circumstance limits their use in many cases. We have already explained the reason of this reversal (p. 91). Copies can, however, be obtained in their proper position if the original drawing is very thin. In that case the back of the drawing is placed against the chromic acid paper, and the light is suffered to shine through it from the upper surface.
Another disadvantage of this process is that the chromic acid paper must be always freshly prepared, as it quickly spoils, and further, there is the difficulty of ascertaining the proper time for exposure. If the time is too short, unchanged chromic acid remains everywhere on the paper, and thus the whole picture is blackened by the aniline. If, again, the time is too long, the light acts gradually through the black strokes of the drawing, reduces the chromic acid, and the paper then remains entirely white in the aniline fumes, as no more chromic acid is present to form aniline colours. These circumstances limit the value of the method.
Lithography. — By lithography is understood a process of printing from a drawing or painting on a prepared stone.