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LEFT PHOTOGRAPHY 233 PHOTOTHERAPY ing it on a solution of silver nitrate, and drying. After printing, the paper is treated with a solution of chloride of gold, which tones it, producing different shades of color, depending on the length of immersion and strength of solution. After toning, the print is fixed in the same manner as the plate, by a solution of hyposulphite of sodium, which removes the undarkened silver salts. Gelatine or collodion is sometimes used in place of albumen in this silver-printing process. All soluble substances have to be washed from the prints after they have been toned and fixed because otherwise the photographs become ultimately discol- ored. The papers known as aristotype, argentotype, and celerotype are gelatine emulsions of chloride of silver spread on paper. Platinum Process. — An image can be obtained in platinum black if the paper is sensitized with ferric-oxalate with which is mixed a solution of chloro-pla- tinite of potassium. The action of light on this paper reduces the ferric salt to the ferrous state, and when the ferrous salt is in solution the platinous salt is reduced by it. By floating the exposed paper on a solution of neutral potassium oxalate, which is a solvent of the ferrous oxalate, the platinum salt in contact with it is immediately reduced to the metallic state, and an image is thus built up. To fix the prints they are immersed in dilute hydrochloric acid, which dissolves away the ferric oxalate, and the oxalate of lime. Ferrojmissiate Process. — The ordinary method of making blue prints. Composite photography, a method of superposing several or many photo- graphs, thus getting a sort of average of the whole and showing the type. If it is a human composite photograph type, then the eyes of each sitter are brought to the same place on the lens, and the exposure for each is very short. The most important advance in pho- tography in recent years has been made in the direction of taking photographs in natural colors. Many scientists have worked on the problem and several proc- esses have been developed. The most remarkable of these was perfected in 1907 by Antoine Lumiere and his sons August^ and Louis, of Paris, who suc- ceeded in taking color photographs on a single plate and in an ordinary camera, with exposures of one second and less. Their process consists in the formation of a color screen on a glass plate by plac- ing on it a layer of microscopic grains of transparent potato starch, in three por- tions colored respectively orange, green, and violet; the plate is then sensitized by coating with a gelatin-bromid emul- sion. After the exposure, the plate is developed by a double process that turns it to a positive and the result is a beau- tiful transparency in the natural colors. PHOTOGRAVURE, a term applied to methods of producing, by photography, plates for printing in a copper plate press. The processes are kept secret. PHOTOPHOBIA, a dread or intoler- ance of light; a symptom more or less present in all inflammations of the eye. It is also met with in many diseases of the nervous system and in many febrile conditions. PHOTOSPHERE, in astronomy, a luminous envelope believed to completely surround the sun within an outer environ- ment of a dense atmosphere. It is from the photosphere that light and heat are radiated. Used more rarely of the fixed stars. PHOTOSTAT, a trade name for direct photographing process. By this means copies of drawings, legal papers, records, etc. may be rapidly produced. A special camera, with special developing and fix- ing attachments, is used in the process. The reproductions are made directly upon the surface of the print paper, which has been coated with a special emulsion. In order that the image will not be reversed the camera is fitted with a reversing prism and a special copying lens. The entire apparatus of the camera and developing plant is self-con- tained. After exposure the paper passes through the developing and fixing proc- ess, and is then cut and dried and is ready for use. The ordinary photostat print is white upon a black background, but by a slight addition to the process, black prints upon light backgrounds are secured. The pro- cess is much used for legal work, dupli- cating policies, in insurance, and pair- ticularly in engineering work, where many copies of the same plans are de- sired. The scale of the drawing made may be easily altered. ^ PHOTOTHERAPY, the application of light rays to the treatment of such dis- eases as tubercular glands, eczema, can- cerous grovrths, lupus epithelioma, and acme vulgaris. In the decomposition of sunlight or artificial white light, it has been found that different rays have different quali- ties, some being calorific, or heat-produc- ing, others producing light without heat. Experimentation has proved that there are rays beyond both extremes of the visible spectrum, called ultra rays. The ultra-red rays produce greater heat than