photoengrave (fotoengrav)

t. [ photo-engraving (fō"tō-en-grāʼving), n. [ φῶς (φωτ), light, + E. engraving.] A common name for many proce es by which a photograph may be made to to afford a plate-matrix from which can be taken prints in ink corresponding to the original of the photographic image. These proce es depend upon the property, po e ed by pota ium bichromate and analogous chemicals, of rendering insoluble, under the action of light, gelatin or some similar body with which they are compounded. By applications of this property, varying according to the proce , a picture or design can be produced on a metal surface, and the blank places etched out with acid; or a matrix in relief can be formed, from which an electrotype plate can be made in or IMG:content-0063.png:[ocr errors] IMG:content-0064.png:[ocr errors] means for printing in an ordinary printing-pre , to the art of making such blocks, and to prints from them; while the term photogravure is commonly applied to a a photograph i cally engraved plate in intaglio from which prints may be taken in a copperplate-pre , to the art of making such an incised plate, and to a print from it. In the Gillet proce a zinc plate coated with asphaltum is exposed beneath a negative, and those portions unchanged by light are di olved. The zinc is then etched. Photographs are reproduced in the form of half-tone plates for use in the upon breaking up the surface of the picture by dark lines in regular series. A gelatin film on which such a series of lines has been photographed is placed between the sensi tized surface which is to receive the impre ion and a positive picture. The resulting print will consist of the subject appearing in half-tone on a ground of lines, and from it a typographical matrix is prepared in the usual ways. (For an example of a half-tone plate, see cut under dekadrachm.) Also called photographic engraving, photo graphic-proce printing, photographic proce . See photo etching (Gillet proce ), and compare heliotypy and photo gravure. photo-epinastic (fō-tō-ep-i-nas'tik), a. [ sensitive film, exposing to the action of light, and fixing or developing the image resulting in the film. A variety of photogenic proce es are now in use for copying mechanical drawings. See blue - printing. photogen ous (fō-toj'e-nus), a. [ Their further studies. Gr. φῶς φῶς (φωτ-), enable them to reconcile photography photographer (fō-tog'ra-fér), n. [ photographical (fo-to-graf'i-kal), a. [ photographically (fo-to-graf'i-kal-i), adv. By n. [ means of, or as regards, photography; as in a photograph. photographometer (foto-gra-fomʼe-tèr), photograph + Gr. μέτρον , measure.] 1. In photog ., an instrument for determining the sensibility of a film employed in photographic proce es, relatively to the amount of radiation, luminous and chemical.-2. A rotating photo- graphic apparatus for recording automatically the angular position of objects around a given point. photography (fo-tog'ra-fi), n. [= F. photo- graphie = Sp. fotografía = Pg. photograph i a = It. fotografia = D. photografie = G. photo- graphie = Sw. Dan. fotografi , photography (in Sp. Pg. It. also a photograph), = NGr. φωτο- γραφία , photography, φῶς (φωτ-), light, + -γραφία , γράφειν , write. Cf. photograph .] The art of producing images of objects by an appli- cation of the chemical change produced in cer- tain substances, as silver chlorid, bromide, or iodide, by the action of light, or more gener- ally of radiant energy. The rays which are in gen- eral most active in this way are those of the upper part of the spectrum, as the blue, violet, and ultra-violet rays. The red and yellow rays produce a much le marked ef- fect on an ordinary sensitive plate; but it has been found po ible to prepare a special gelati no bromide plate which is highly sensitive even to the le refrangible rays, as those in the infra-red region of the spectrum. (See spec- trum .) Photography rests on the fact that silver nitrate and various other chemicals are decomposed by certain solar rays and reduced, becoming dark or black, or in other ways affected, according to the intensity and amount of actinic rays received on them. The proce consists (1) in properly exposing a surface made sensitive to actinic rays to a projected image of the object to be reproduced; (2) in rendering visible if merely latent, or in coloring or toning, the reproduction of this image; (3) in removing the their theory of photogen ous fermentation with the hypo the sensibility of those parts of the surface which have not been sis of the oxidation of a phosphorated substance, as proposed by some biologists. Nature, XXXVIII. 512. Aud photogeny (fo-toj'e-ni), n. [ photogravure. photogram (foto-gram), n. [ IMG:content-0065.png:[blocks in formation] f = NGr. φωτογράφος, a pho a negative. The parts affected by light through the transparent parts of the negative harden, while those which are protected from the light remain sticky, so that any fine powder dusted over will adhere to them, while having no hold on the hardened parts. If a phosphorescent powder is dusted on this positive, and the plate is then exposed to strong light, there will result a picture appearing luminous in the dark. acted on, and in fixing permanently the image produced; and (4), if the image obtained is a negative, as in the majority of proce es, in the mechanical production of positive copies from it. The knowledge of the principle on which mists, who discovered that silver chlorid exposed to the sun's rays became black. Wedgwood and Davy in 1802 attempted to apply this fact to artistic purposes by throwing the shadow of an object on a sheet of white paper, or, preferably, of leather, covered with a solution of silver nitrate and exposed to the sun's rays, but they were unable to fix the pictures. About 1814 Niepce, a French man, discovered a method of producing pictures on plates of copper or pewter covered with a sensitive resinous substance called bitumen of Judea, and also of rendering them permanent. This proce he called heliography. Niepce a ociated himself with Daguerre, who elaborated, about1838, from the former proce the one which bears his name. (See daguerreotype.) This was soon superseded by various proce es, especially the calotype proce (see calotype ) of Fox Talbot, first patented in 1841, who revived Wedgwood's proce of obtaining pictures on sensitized paper, and the collodion proce (see collodion ), first suggested by M. Le duce a positive from a negative, the latter is placed in con tact with a surface which has been rendered sensitive to light, and is exposed to the influence of light, which penetrates the negative and affects the parts of the underlying surface opposite the lights of the picture, while the parts the opaque parts of the picture are protected. operation for obtaining a positive from a negative is called printing. Modifications are constantly being introduced in photography. One of the most important has been the carbon proce , popularized by Swan of Newcastle, according to which a solution of gelatin and pota ium bichromate (the latter being the sensitizing agent) is mixed with a pigment, and applied as a coating to a sheet of paper. The positives are printed in the ordinary way on the black cake, or ti ue as it is called, thus produced, and become visible and permanent by washing, as the pigment-coating is rendered more or le insoluble by the effect of the light pa ing through the negative. The autotype proce , invented by Johnson, is a simpler method of carbon-printing than the carbon proce proper, but the principles involved are the same. One of the most important developments of the art is the so-called instantaneous photog photograph (foto-graf), v. t. [ tive plates and the shortne of the duration of exposure, photography scenes, motions, etc., are reproduced and registered which are too rapid or evanescent to be distinguishable by the eye. For various mechanical methods of multiplying photographic pictures, see photo - electrotype, photo - engraving, pho togalvanography, photoglyphy, photogravure, photolithogra phy, and heliotypy. photogravure (fōtō-grā-vūr'), n. [ lines remain as bare copper. The plate is then etched. In the Fox-Talbot proce the gelatin print is transferred to copper which has had a grain given to it by sprinkling the surface with powdered resin and then warming it. (See aquatint.) The plate is then etched with ferric acid, which renders the opaque portions of the gelatin film insoluble and impermeable. The acid should be weak and kept in motion during the biting, until the uncovered parts have been sufficiently attacked. To increase the regularity of the erosion, the plate should first be im mersed in a weak solution of copper sulphate. In the Woodbury proce , which resembles the Goupil proce , a gelatin picture in relief is applied under pre ure upon a plate of soft metal, and is repeated on the metal in relief and depre ion. The mold thus formed is filled with pigmented gelatin, over which a sheet of paper which is to receive the picture is placed, and subjected to a level pre ure in order to force out the superfluous gelatin. The depre ed parts, which represent the dark parts of the picture, retain the most gelatin, and when the paper is lifted it raises the gelatin from the mold in such a manner that it forms a picture in low relief. In order to obtain a grained surface which will hold printing-ink, pounded gla may be mixed with the gelatin. photogravure (foto-grā - vūr "tō-gra-vūr′), v. t.; pret. and perphotogravured , ppr. photogravuring . [ vure.