n. The science which relates to the action of light on sensitive bodies in the production of pictures, the fixation of images, and the like. The art or process of producing pictures by this action of light. 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.