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NatureIf a negative photograph, or a piece of lace or a leaf, be placed over the prepared paper, and put in sunshine, in a few minutes a perfect impression of the object is obtained. The light darkens the colour of the bichromate, and renders it insoluble in water, while the yellow colour washes out from the parts protected from the light by the lace or leaf, or negative photograph, as the case may be. But pictures of this kind have little or no practical value; for although the lights are good enough, the deep black shadows are only represented by a tawny shade. Some eighteen months ago a process was patented for deepening these photographs by treating them with gallic acid and a salt of iron, which went by the name of ' Sella's process.' I tried this process at the time according to the specification of the patent, but failed to make one satisfactory specimen. They wanted everything that a good photograph should have,—pure lights, clear halftints, and deep shadows,—and as I found that others had not been more successful, I abandoned my experiments.
W. M'Craw, "On a New, Cheap, and Permanent Process in Photography", Twenty-Eighth Meeting of the British Association for the Advancement of Science, Section A - Mathematical and Physical Science, The Athenaeum, No.1618, Oct. 30, 1858, p.554