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.
PHOTOGRAVURE
Pho`to*grav"ure, n. Etym: [F.]
Defn: A photoengraving; also, the process by which such a picture is produced.
PHOTOHELIOGRAPH
Pho`to*he"li*o*graph, n. Etym: [Photo- + heliograph.] (Physics)
Defn: A modified kind of telescope adapted to taking photographs of the sun.
PHOTOHELIOMETER
Pho`to*he`li*om"e*ter, n. [Photo- + heliometer.] (Astron.)
Defn: A double-lens instrument for measuring slight variations of the sun's diameter by photography, utilizing the common chord of two overlapping images.
PHOTOLITHOGRAPH
Pho`to*lith"o*graph, n. Etym: [Photo- + lithograph.]
Defn: A lithographic picture or copy from a stone prepared by the aid of photography.