... a solution of the ammonio-citrate of iron. This solution should be sufficiently strong to resemble sherry wine in color. Expose the paper in the usual way, and pass over it very sparingly and evenly a wash of the common yellow ferro-cyanate of potass. As soon as the liquid is applied, the negative picture vanishes, and is replaced by a positive one, of a violet blue color, on a greenish yellow ground, which at a certain time possesses a high degree of sharpness, ... — The History and Practice of the Art of Photography • Henry H. Snelling
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