f gelatin, in which silver bromide is suspended (silver iodide
is sometimes added also), over a glass plate or celluloid film
and allowing it to dry. When the plate so prepared is placed in
a camera and the image of some object is focused upon it, the
silver salt undergoes a change which is proportional at each
point to the intensity of the light falling upon it. In this
way an image of the object photographed is produced upon the
plate, which is, however, invisible and is therefore called
"latent." It can be made visible by the process of developing.

To develop the image the exposed plate is immersed in a
solution of some reducing agent called the developer. The
developer reduces that portion of the silver salt which has
been affected by the light, depositing it in the form of black
metallic silver which closely adheres to the plate.

The unaffected silver salt, upon which the developer has no
action, must now be removed from the plate. This is done by
immersing the plate in a solution of sodium thiosulphate
(hypo). After the silver salt has been dissolved off, the plate
is washed with water and dried. The plate so prepared is
called the negative because it is a picture of the object
photographed, with the lights exactly reversed. This is called
fixing the negative.

2. _Preparation of the print._ The print is made from paper
which is prepared in the same way as the negative plate. The
negative is placed upon this paper and exposed to the light in
such a way that the light must pass through the negative before
striking the paper. If the paper is coated with silver
chloride, a visible image is produced, in which case a
developer is not needed. The proofs are made in this way. In
order to make them permanent the unchanged silver chloride must
be dissolved off with sodium thiosulphate. The print is then
toned by dipping it into a solution of gold or platinum salts.
The silver on the print passes into solution, while the gold or
platinum takes its place. These metals give a characteristic
color or tone to the print, the gold making it reddish brown,
while the platinum gives it a steel-gray tone. If a silver
bromide paper is used in making the print, a latent image is
produced which must be developed as in the case of the negative
i