[Gk. phos, light; and graphein, to write.] The science or art of taking representations of objects by the action of light on a prepared surface. The surface, consisting of metal, glass, paper, or other suitable substance, is prepared by being coated with collodion or gelatine, and sensitized with the chlorides, bromides, or iodides of silver, or other salts sensitive to light. The picture obtained in the camera by means of exposing one or other of these sensitive surfaces to the light cannot be seen when the plate is removed, but by pouring over it a mixture of ferrous sulphate or pyrogallic acid it comes out little by little. This is called developing the picture. Any excess of unchanged sensitive salt is then dissolved off with sodium hyposulphite or other suitable reagents, and in this way the negative image is fixed, from which any number of positive prints may be taken, which are washed, toned, fixed, and then mounted. - Photogravure, a print from a copper or steel plate. The picture is produced on the plate by photography, and bitten in with acid instead of engraving. - Photo-lithograph, a lithographic picture or copy from a stone prepared by the aid of photography.