time in being
thoroughly desiccated. We discard the common alum which we found liable
to produce a slight reticulation.

Two defects are complained of by the beginners, viz., the want of
adherence of the deep blacks, and, especially, the isolated and fine lines
when the picture is a reproduction of an engraving, a drawing, etc., and
the liability in half tone pictures of the delicate details being washed
out. The first defects are avoided by pouring a solution of boric acid on
the transitory support before applying the tissue and developing at a low
temperature with salted water. The second from an imperfect knowledge of
the properties of gelatine acted on by light in presence of a salt of
chromic acid. One should bear in mind that the degree of solubility of
gelatine so acted on, as also its degree of impermeability--which is
important in certain processes of photogravure--is proportionate to the
degree of insolation; thus, when not impressed, bichromated gelatine
dissolves in water heated to about from 25 to 30 deg. C. (77 to 80 deg.
Fahr.), and when acted on between 30 and 100 deg. C. (86 to 112 deg.
Fahr.), according as to the degree of insolation, that is, of reduction of
the chromic salt, the latter temperature being that of insolubility of the
parts the most acted on. The very delicate half tints do not, generally,
stand a temperature higher than 35 deg. C. (95 deg. Fahr.), and,
therefore, as the degree of insolubility of the various parts cannot be
ascertained, a priori, it is advisable during the development to increase
gradually the temperature of the water from this degree, and not to exceed
45 deg. C. (113 deg. Fahr.), in order to obtain the most perfect result
from a negative of good intensity. Indeed, by placing the supports on a
rack and immersing the whole in water heated to 30 to 35 deg. C. (86 to 95
deg. Fahr.), the image will clear up by itself to perfection in a certain
period. This method is excellent for proofs in lines. Those from the
grained negatives employed in photogravure are still more perfectly
developed in a tepid solution of potassium sulphocyanate, since the
impressions wholly consist of insoluble parts (the lines) and gelatine not
acted on.

_Retouching.--_The retouches are easily made. They should be done before
transferring when working by the double transfer process.

The transparent spots, and any parts which should be altered, are
retouched with the material of the tissue