PHOTOGRAVURE, firto-gra-viie, a process of engraving in which, by the aid of photography, subjects are reproduced as plates suited for printing in a copper-plate press. The process known as heliogravure is essentially the same. See Pficao-Exclitivilic.
that depart ment of photo-engraving in which the plates are in intaglio. See PHOTO-ENGRAVING.
an instrument for observing transits of Venus and other solar phenomena, consisting of a telescope mounted for photography on an equatorial stand and moved by suitable clockwork.
a method of producing by photographic means designs upon stones or zinc or aluminum plates, from which impressions may be obtained by lithographic process. The first requisite for the production of a good result by this process is a suitable original. The drawing should be made with perfectly black lines throughout, no matter how thin the lines are; the scale of reduction should not he too great; the best proportion is ob tained when the drawings are made about one third larger than the required block; the paper used should be white and smooth in texture.
The negative for a line-block is made prefer ably by the wet-plate or collodion process, be cause of the facility with which these plates can be intensified and the clearness of the lines.
Asser of Amsterdam was the first to put photo lithography to practical use, but probably it is to Osborne of Melbourne that we are indebted for the modifications which made it the process now employed by every map-maker in the world. A sheet of suitable paper is coated by floating on a solution of bichromated gelatine or albumen; printed under a negative and inked either by a roller or, better still, by spreading the ink evenly and passing the paper through the press once or twice as if drawing a proof. The inked sheet is then laid face down on warm water if gelatine has been employed and cold if albumen. The gelatine, where light has not affected it, swells and dissolves, leaving ink only where light has acted, the parts repre senting the dark lines of the original. A spray with water or even a slight wash with a sponge makes it ready, after drying, for transferring to the stone or plate and the quality of the work will depend on the care given to the preparation of the transfer.
Photo-lithography is the principle used in offset printing. See OFFSET-PRESS under PRINTING PRESSES; also PHOTO-ENGRAVING.