2. The art or process of producing pictures by this action of
light.
Note: The well-focused optical image is thrown on a surface
of metal, glass, paper, or other suitable substance,
coated with collodion or gelatin, and sensitized with
the chlorides, bromides, or iodides of silver, or other
salts sensitive to light. The exposed plate is then
treated with reducing agents, as pyrogallic acid,
ferrous sulphate, etc., to develop the latent image.
The image is then fixed by washing off the excess of
unchanged sensitive salt with sodium hyposulphite
(thiosulphate) or other suitable reagents.