of aggregating together like ordinary crystals. I have constructed a
diagram of colored card, which will perhaps more clearly demonstrate
the relation of the different constituents. The lower portion (Fig. a)
represents a section of the glass plate or support, the collodion film
(Fig. b) having upon its surface a thin layer of bromo-iodine silver
(Fig. c), which, when exposed to a well-lighted image, as in a camera,
changes into different gradations of sub-bromide and sub-iodide, as
indicated by irregular, dark masses in the film. The dotted marks
immediately above these are intended for the silver deposit (Fig.
d)--clusters of granules, more abundant in the well lighted and less
in the shaded parts of the picture, corresponding to the amount of
sub-bromide and iodide beneath.

[Illustration: SECTION OF SENSITIVE PLATE AFTER EXPOSURE AND DURING
DEVELOPMENT.

d Silver deposit--Image, c Sub-bromide and sub-chloride (gradations of),
b Collodion film--Substratum, a Section of glass plate--Support.]

The next point to consider is that of intensification--a process seldom
required in positive pictures, and would not be needed so often in
negatives if there was enough free silver nitrate on the plate during
development. The object, as we all know, in a wet-plate negative is to
get good printing density without destruction of half-tone. It is a
rule, I believe, in an over-exposed picture to intensify after fixing
the image, and in an under-exposed picture to intensify before fixing.
Whichever is done the intention is similar, namely, to intercept in a
greater degree the light passing through a negative, so as to make a
whiter and cleaner print. The usual intensifier--and, I suppose, there
is no better--is pyrogallic acid, citric acid, water, and a few drops of