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LEFT PHOTOELECTROTYPE 232 PHOTOGRAPHY photographic dark rooms in which white light is excluded and red lamps or win- dows employed. The action of light produces on photographic plates and bro- mide papers an effect which is not imme- diately visible, but can be made so by further action of a chemical known as a developer. PHOTOELECTROTYPE, a block made mainly with the aid of photography and of the electrotyping process, and which can be printed with type like a woodcut. A photographic negative of the subject required is printed on a film of gelatine which has been treated with bichromate of potash, to render it sensitive to the action of light. Those parts on which the light has not acted are soluble in water, and are washed away, leaving the printed parts that are insoluble in relief. From this relief a mold is taken, and an electrotype made in the usual way. Un- less special means are taken to translate the half-tones of the photograph into line or stipple, this process is only available for reproducing drawings, etc., in black and white. PHOTOENGRAVING, a term applied to producing printing blocks or plates by photography. The most commonly em- ployed process is to coat a metal plate with a thin film of asphaltum, and ex- pose it to light under a reversed positive. The picture is next developed by dissolv- ing away the parts of the asphaltum not acted upon by the light, and the plate is subsequently etched in the usual way. This process is sometimes called photo- aquatint. The second method is more elaborate. A film of bichromatized gela- tine, on a sheet of glass or a copper plate, is exposed under a photographic negative, and the unprinted portions which are soluble in water washed away, leaving the printed parts in relief. The plate with the relief is next coated with a film of silver by electro-deposition, and placed in an ordinary electrotyping bath, in which it is allowed to remain till a shell of copper from one-sixteenth to one- eighth of an inch thick (according to size of plate) is formed. This, after the rough excrescences have been removed by filing, becomes the printing plate. It can be worked upon by an engraver, if necessary, to remove photographic de- fects, and is printed on a copper-plate press. When a relief block is required, a reversed negative is used to print from, and the etching is carried to a much greater extent. These processes answer for^ subjects in black and white, as well as in colors. This is also the system by which conversion of the half-tones of a photograph into an ordinary printing block or plate has become so emi- nently successful. See Photoelectro- TYPE; Photogravure, PHOTOGRAPHY, the art of produc- ing permanent pictures by means of the chemical action of light on sensitized sur- faces. The first to produce permanent pictures by the chemical influence of light was M. Niepce at Chalais-sur-Mer, in 1814, and his idea of the development of a latent image by treatment after exposure lies at the basis of photography. Niepce and Daguerre discovered that they were conducting experiments of a kindred character, and in 1829 entered into partnership. Daguerre, with Niep- ce's son Isidore, carried on this work in 1839, and invented the process known as the daguerreotype. The introduction of collodion in 1857 was a great step in photography. The collodion process is divided into five stages: (1) The preparation of a clean glass plate by coating with collodion to which bromide of cadmium and either iodide of potassium or iodide of ammo- nium has been added. (2) The sensitiz- ing of the collodion by immersion in a bath of nitrate of silver, and distilled water. (3) The production of a latent image by exposing the sensitized plate in the camera. (4) The development of latent into visible image by immersing the plate in a solution of sulphate of iron or pyrogallic acid, to either of which some acetic or citric acid is added. (5) Fixing of the permanent image by im- mersion of the plate in some solvent of those parts of the sensitive surface upon which the light has acted. This solvent for wet plates is cyanide of potassium, but for more modern processes hyposul- phite of sodium is employed. On account of the trouble of preparing the wet col- lodion plates, the dry plates, made by the gelatine-bromide process, have almost universally taken their place. Dry plates made^ by some form of this gelatine- bromide process are manufactured on a large scale. When properly made they are^ much more sensitive than the col- lodion plates and will keep good for years, and can be developed months after exposure. The positive prints are made by the action of light, passing through the developed plate, on a sensitized pa- per, over which the plate is placed. The silver, platinum, and ferro-prussiate pa- pers are the most used. Silver Process. — Suitable paper is coat- ed on one side by a thin layer of albu- men, to which chloride of sodium or of ammonium has been added. The old method of dispensing with albumen and using paper which is salted only, is re- viving. The paper is sensitized by float-