hich falls upon the sensitive plate and
forms an image there will change the silver chloride and produce a
hidden image. If the plate is then removed from the camera in the
dark, and is treated as described in the preceding Section, the image
becomes visible and permanent. In practice some gelatin is mixed with
the silver salt, and the mixture is then poured over the plate or film
in such a way that a thin, even coating is made. It is the presence of
the gelatin that gives plates a yellowish hue. The sensitive plates
are left to dry in dark rooms, and when the coating has become
absolutely firm and dry, the plates are packed in boxes and sent forth
for sale.

Glass plates are heavy and inconvenient to carry, so that celluloid
films have almost entirely taken their place, at least for outdoor
work.

123. Light and Shade. Let us apply the above process to a real
photograph. Suppose we wish to take the photograph of a man sitting in
a chair in his library. If the man wore a gray coat, a black tie, and
a white collar, these details must be faithfully represented in the
photograph. How can the almost innumerable lights and shades be
produced on the plate?

The white collar would send through the lens the most light to the
sensitive plate; hence the silver chloride on the plate would be most
changed at the place where the lens formed an image of the collar. The
gray coat would not send to the lens so much light as the white
collar, hence the silver chloride would be less affected by the light
from the coat than by that from the collar, and at the place where the
lens produced an image of the coat the silver chloride would not be
changed so much as where the collar image is. The light from the face
would produce a still different effect, since the light from the face
is stronger than the light from the gray coat, but less than that from
a white collar. The face in the image would show less changed silver
chloride than the collar, but more than the coat, because the face is
lighter than the coat, but not so light as the collar. Finally, the
silver chloride would be least affected by the dark tie. The wall
paper in the background would affect the plate according to the
brightness of the light which fell directly upon it and which
reflected to the camera. When such a plate has been developed and
fixed, as described in Section 121, we have the so-called negative
(Fig. 83). The collar is very dark, the black tie and gray coat w