he design, well defined in all its details, is visible on the back
of the paper, which requires an insolation of about two minutes in clear
sunshine, and from eight to ten times longer in the shade. In cloudy
weather the exposure to light is necessarily very long.

As explained before, the luminous action, by reducing the chromic salt in
presence of certain organic substances, causes the latter to become
insoluble; consequently if, on its removal from the printing frame, the
proof be soaked in cold water, for, say, ten minutes, and, placing it on a
glass plate or a smooth board, gently rubbed with a brush or a soft rag,
the parts of the albumen or gum arabic preparation not acted on will
dissolve, leaving behind the black image standing out on the white ground
of the paper. This done, and when the unreduced bichromate is washed out
in two changes of water, the operation is at an end.

As to the theory of this and similar processes, the insolubilization of
the bichromate organic substance acted on by light was formerly attributed
to the oxidation of the substance by the oxygen evolved during the
reduction of the chromic salt into chromic oxide; but from the fact that
oxidation generally tends to destroy organic matters, or to increase their
solubility, it is more probable that it results from the formation of a
peculiar compound of the substance with chromic oxide (J. W. Swan);
moreover, gelatine imbued with an alkaline bichromate, then immersed first
in a solution of ferrous sulphate and afterwards in hot water, is
insolubilized with formation of chromium trioxide, Cr2O7K2+SO4Fe =
SO4K2+C2O4Fe+C2O3 (Monckhoven). A similar but inverse action occurs, as
shown by Poitevin, when gelatine rendered insoluble by ferric chloride
becomes soluble by the transformation, under the influence of light, of
the ferric salt into one at the minimum.

The writer has improved the above process by simplifying the modus
operandi as follows:

Instead of compounding the preparation with gum arabic and the coloring
matter, the albumen is simply clarified by beating the whites of eggs to a
froth, etc., and the paper is coated by floating for one minute, then hung
up to dry in a place free from dust.

If the reader has any objection for albumenizing his own paper, he can use
the albumen paper found in the market for the printing-out silver process
generally employed by photographers.

The paper is sensitized from the back with the potassi