photographist

[Eng. photograph; -ist.] A photographer. phô-tô-grăph-om-e-ter, s. [Eng. photograph; o connective, and Eng. meter.] Photography: An instrument for determining the sensibility of each tablet employed in the photographic proce , in respect to the amount of luminous and chemical radiation. phō-tog'-ra-phy, s . [Pref. photo- , and Greek grapho to write; Fr. photog rap hie .] The art of producing pictures by the action of certain sensi be said to have sprung from the discovery, some tive substances, under the influence of light. It may three hundred years ago, that the luna cornea of the alchemists-i.e., fused silver chloride-would darken on exposure to light. Nothing more was known until in 1777 Scheele, the Swedish chemist, noticed that the power which produced this darkening resided chiefly in the violet end of the solar spectrum. In 1802 Thomas Wedgwood published of taking profiles, upon paper or white leather treated with nitrate of silver, and exposed to the light of the sun under the object to be represented. For many years no method was known known of fixing the picture, i. e., of di olving away the unaltered sensitive salt; but the difficulty was eventually overcome me by Sir John Herschel, when he suggested the use of hyposulphite (thiosulphate) of soda, a salt now used for the same purpose. M. Niepce was the first worker with bitumen of Judæa, which loses its solubility in certain media when exposed to the light, and his method has since been enormously developed as the basis of photo etching, and many other proce es. The year 1839 was one of paramount importance in the history of photog raphy, for then Henry Fox Talbot published his calotype proce , in which paper, having on its sur face chloride of silver. was exposed in a camera obscura (q. v.), and the image developed by a solution of gallic acid. The discovery of this kind of development, which marks an epoch in the history of photography, is due to the Rev. J. B. Reade, The pictures so produced were negatives (q.v.), and from them positives were obtained by exposing to light, under them, another sensitive sheet. The substitution, in the next year. of silver iodide for chloride, greatly improved the proce , which was now thoroughly workable, and means many beautiful results have been obtained. In the same year, Mungo Ponton observed the sensitivene to light of paper containing bichromate of potash. This phenomenon, the true nature of which was explained by Becquerel in 1840, has given birth to the carbon proce , the Woodburytype (q. v.), and many others. The world-famous Daguerreotype proce was also published in 1839, a film of silver iodide on a plate of silvered copper being the sensitive material, the pictures on which were devel oped by the vapor of mercury. This proce is still used for making photographs from which accurate measurements are to be taken. In 1850 the art of photography was greatly advanced by the introduction of Mr. Scott Archer's proce , in which the sensitive iodide and bromide of silver are held in a film of collodion, on gla , the image being developed with pyrogallic acid, or a ferrous salt. The next great step forward was was the adoption of alkaline development for dry plates. The collodion proce (q.v.) still holds its own for many purposes, and was generally employed until a few years ago, when the art was once more completely revolutionized by the introduction of gelatine, which may be photomechanical spread either upon gla or paper, as a medium for holding the sensitive salts. The sensitivene of these gelatine plates is so great that photographs of expre trains in motion, leaping horses, and birds on the wing are of everyday occurrence. [KINETOGRAPH, MUTOSCOPE, VITASCOPE.] The so-called "New Photography," by means of which it is po ible to obtain a shadow picture of the bones in a living body, or of a foreign metallic object, such asa bullet, imbedded in the flesh, is treated of herein under its more appropriate title RADIOGRAPHY (q.v.). The application of photography to astronomy has been attended, of late years, with truly remarkable results, for we have now pictures of every object in the heavens, from the nebula in Orion to the spots upon the face of the sun himself. Photographs in color, upon silver chloride, and even photographs of sound waves have been exhibited. [CALOTYPE, CAMERA-OBSCURA, CARBON-PRINTING, COLLODION PROCESS, HELIOTYPE, PLATINOTYPE, POSITIVE, SILVER-PRINTING, STANNOTYPE, WOODBURYTYPE.] phō-to-gra-vüre, s. [Fr.] A term applied to methods of producing, by photography, plates for printing in a copper-plate pre . The proce es are kept secret; but, in one of them, the translation of photographic half-tones into the corresponding grain required for printing, is said to be effected by the aid of substance which crystallizes when exposed to light, the size of the crystals depending upon the amount of light they receive. Such a substance, exposed under a negative, will give a surface, the grain of which will exactly correspond with the lights and shades of the picture, and from which an electrotype can be made for printing purposes. phō-to-gra-vüre', v . t . & i . [PHOTOGRAVURE, 8.] To produce by the method of photogravure. phō-tō-hê-li-o-graph, s . [Pref. photo- , and Eng. heliograph (q. v.).] An instrument made by Dallmeyer, for photographing transits of Venus. It consists of a telescope, mounted for photography corona to rial stand anche suitabl has an object-gla of four inches in diameter and five feet focal length. and actuated by IMG:content-1006.png:[blocks in formation]