PHOTOGRAPHY, the science and art of producing pic tures by the action of light on chemically prepared (sensitized) materials. (Gr. cbws, light, and 7packtv, to write.) The practice of photography depends upon the sensitiveness of silver compounds to light. When silver bromide, dispersed in a suitable medium such as gelatine, is exposed to light, it under goes a change as a result of which it becomes developable. When, after exposure, it is treated with a solution of a particular type having reducing properties, termed a developer, the exposed bro mide is reduced to metallic silver and forms an image correspond ing to the intensity of light to which it was exposed. (See fig. I.) After development, the unexposed silver bromide is removed by a suitable solvent which does not attack silver but dissolves the bromide and the picture is then permanent and is said to be fixed. The picture thus obtained is a negative; that is, the light parts of the object are the blackest in the picture and the dark parts, which have affected the silver bromide least, are the brightest.
In order to obtain a positive picture, in which the light and shade corresponds to that of the original subject, the negative is printed. The negative is placed in contact with another piece of sensitive material which is exposed to light through the nega tive, and this gives in turn a positive image. (See Plate I.) The operations required to produce a photograph are thus : I. Preparation of the sensitive negative material 2. Exposure in a camera 3. Development; with the after processes, fixation, washing, and drying.
4. Preparation of the sensitive positive material 5. Printing 6. Development, fixation, washing, and drying of the print. In the early days of photography all of these operations were carried out by the photographer, but at the present time the sen sitive materials, both negative and positive, are almost always made by manufacturing companies. In many fields of photography only the exposure, including of course the selection of the sub ject, is actually carried out by the photographer himself, the re maining operations being performed by others.
A wide variety of negative materials, cameras, and printing processes , are used in photographic operations, their selection depending upon the particular branch of photography under con sideration.
The chief branches of photography are : Amateur Photography including both the casual taking of photographs for record or amusement and the serious pursuit of pictorial photography as an art.
Motion Picture Photography (q.v.) both professional and amateur.
Radiography, photography by means of X-rays, for medical, scientific and industrial purposes.
Applied Photography, including many uses of photography in connection with the arts and sciences, such as aerial photography, photomicrography, astronomical photography, photographic re cording, etc.
Negative materials are made by preparing a precipitate of sil ver bromide containing a small amount of silver iodide held in a medium containing enough gela tine to enable it to be set to a jelly when cooled. This suspen sion is termed the emulsion. The melted emulsion is coated upon a support which may either con sist of glass, in which case plates are produced or may be a flexi ble cellulose compound, in which case the product is termed film. After setting by chilling and drying, the material is cut to • • suitable sizes and packed in light-tight containers.
Films are packed as flat films, for use in holders similar to those used for glass plates, cartridge or roll films protected with paper to make them suitable for loading in the daylight, film packs, and as long ribbons for use in the taking of motion pictures.