HELIOGRAPHY

(n.) An early photographic process invented by Nicephore Niepce, and still used in photo-engraving. It consists essentially in exposing under a design or in a camera a polished metal plate coated with a preparation of asphalt, and subsequently treating the plate with a suitable solvent. The light renders insoluble those parts of the film which is strikes, and so a permanent image is formed, which can be etched upon the plate by the use of acid.