PHOTOLITHOGRAPHY. In later years photog raphy has been made a valuable adjunct to lithography. One of its applications is photo lithography, by means of which pen drawings are faithfully reproduced at comparatively -mall ex pense. Another result k the application of the half-tone or Aleisenbach process to lithography, for monochromes, and chromolithographs. in which ease a photographic reprodnetion from the original on stone forms the basis for the several color plates, taking the place of the key. subject to the manipulation of the expert chromolithog raphcr.
of the several methods in use to achieve a photographic reproduction on stone, the oldest one, invented by Niepce in 1833, is based um the property of asphaltum dissolved in ell of laven der to become insoluble when exposed to actinic action. Thus when a half-tone negative is super imposed upon a stone coated with an asphalt so lution, in perfect contact, and exposed to the sun's rays, the asphalt solution will only be affected by such light as can pass through the negative. Dissolving the parts not affected in this manner, by means of turpentine, will pro duce a positive on stone, which, having in its com position the necessary ingredient, grease, is pre pared for printing purposes in very much the same manner as drawing on stone.
The method most generally employed. however, is the albumen process. Albumen in solution with bichromate of potassium has the same properties as the asphaltum solution. it is more sensitive to the chemical action of light. achiev ing the results desired in a much shorter space of time. After exposure the stone is immediately covered with printing-ink to prevent further ac tion. The parts that remain soluble are washed away with water, the ink adhering only to the parts that have become insoluble, thus ( reating the conditions essential for lithographie purposes.
The time-color process is also applied to some extent in lithography, reproducing on stone the three negatives necessary for this process by either of the methods described. Another iihoto lithographic process, which, though still in its ex perimental stage. deserves mention. is tl e grain proeess. The object in view is to substitute tor the geometrical pattern characteristic of half tone work. a somewhat irregul or: in fnish which more closely resembles tl e re-.alts t f a 'rayon drawing on stone. Tome v ry f. tr results have been achieved in this dire, tion.