M. Regnault has laid before the Academy of Sciences, at Paris, some

photographic specimens on paper, obtained by M. Blanquart-Evrard, by
a modification of the usual process. In the preparations hitherto
described, one part of the process presented serious difficulties,
namely, that of the use of gallic acid in order to produce the
impression. It happened frequently, that a proof taken in too mild a
light, or of too large dimensions, could not receive the necessary
force before disappearing, as it may be said, under the uniform colour
produced by the mixture of the gallic acid with the aceto-azotate of
silver, with which the paper is imbued. After having ascertained that
the gallic acid produces this uniform colour on the impression, only
because it is combined in small quantity with the aceto-azotate of
silver, M. Blanquart-Evrard removes all the difficulty. After taking
the proof from the camera obscura, he plunges it into a vessel of
large dimensions, covered with a layer of one centimètre of gallic
acid of cold saturation. The bath is agitated during the immersion;
and the action may be thus prolonged until the impression has obtained
the necessary force to secure a good result. The proof is then
washed, and the gallic acid is replaced by a solution of bromure of
potassium, or chloruret of sodium, in which it is left for about a
quarter-of-an-hour.[H]

The chromatype, discovered by Mr. Hunt, consists in washing good
letter-paper with the following solution:--

Bi-chromate of potash 10 grains
Sulphate of copper 20 grains
Distilled water 1 ounce

Papers prepared with this are of a pale yellow colour; they may be
kept for any length of time without injury, and are always ready for
use. For copying botanical specimens or engravings, nothing can be
more beautiful. After the paper has been exposed to the influence of
sunshine, with the objects to be copied superposed, it is washed over
in the dark with a solution of nitrate of silver of moderate strength.
As soon as this is done, a very vivid positive picture makes its
appearance; and all the fixing these photographic pictures require is,
well washing in pure water.