CONTRIBUTIONS TO PHOTOGRAPHY. 121
solution of pyrogallol without free silver nitrate was made known by
Mudd, Wardley, and Wharton Simpson, in 1861. All the conditions
for the introduction of alkaline development were therefore ripe, and
the important discovery was announced by Russell in 1862, and
about the same time, but independently, by Leahy.
The importance of alkaline development is too well known to
photographers to require specially enlarging upon. It will suffice to
say that the powerful developer given to the world by our late
member put dry plate photography on an entirely new basis. Want
of sensitiveness could be overcome by increased developing power—
in fact the reducing action of ammonium pyrogallate is so great that
it tends to fog the plate unless restrained by suitable means. In
connection with the general theory of the development of the
photographic image this discovery was of the highest order of impor-
tance, and its bearings were fully recognized by Col. Russell. In
the old process of acid development the image is built up by the
deposition of metallic silver from the developing solution, whereas
in alkaline development the image is formed chiefly out of the silver
reduced in the film itself. For the full establishment of this most
essential distinction between the two kinds of development we are
also indebted to our late member. In the course of a controversy
carried on in 1865 with Carey Lea, of Philadelphia, Russell proved
his point most convincingly by a beautifully devised experiment.
He showed that if an ordinary collodion negative after development
is treated with nitric acid, the whole of the picture is dissolved off,
and the plain film left in its original condition; the silver image is
here simply raised in relief upon the surface of the film, and the acid
dissolves it off. He then showed that a negative dry plate developed
by the alkaline method, and treated with nitric acid as before, gave a
transparent positive of the picture sunk in depression; the silver
image is in this case imbedded in the film instead of being raised
upon its surface, and the acid accordingly dissolves it out, leaving its
counterpart in depression. This proof not only convinced Russell's
antagonist and the photographic world at large of the truth of his
views, but the experiments led to that which is so dear to all
Englishmen, and which is in the minds of many the only true
measure of success, viz., a practical result. He published the details
of a method in 1868, based on the foregoing experiments, by means
of which a reversed negative, or a transparent positive, could be
obtained from a dry-plate negative by treatment with nitric acid after
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