negatives, reversing the darks and lights of the image or object exposed on it, but Herschel also contrived a version, though more complex, to produce positives, which he hoped would aid in his ambition to achieve images of full natural colour. Its difficulties were overcome by Henri Pellet in 1877 in his gum arabic iron cyanofer direct positive photographic tracing method, which he commercialised.

Process

Herschel's formula and method

In a typical procedure, equal volumes of an 8.1% (w/v) solution of potassium ferricyanide

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