528 LITHOGRAPHY one color. Very frequently 10 or 15 separate stones are employed, and in some very elabo- rate prints as many as 30 or 40, some colors being printed over others to produce variations of tint and shading. In the first place a draw- ing is made which contains the general outlines of the position of the different colors. On the second and third stones the general effect of the drawing is worked in ; these are printed in neutral colors, as a pearl gray or faint sepia. Each succeeding stone is charged with its own special tint, brown, blue, green, or yellow, as the case may be; and the last one contains the sharp dark touches, whether of shade or outline, which give character and expression to the whole. Upon the skill with which these colors are arranged, and upon the ac- curacy with which each falls exactly into its proper place, depend the value of the whole work. The misplacement of a single color to the extent of the fiftieth part of an inch might mar the whole. This involves the necessity of the utmost accuracy in the drawing upon each stone, and also in the placing of the paper in its exact place at each impression. The difficulty of this last is much increase^ by the fact that the sheet of paper must be dampened at each impression, whereby it expands per- ceptibly, and dried, when it contracts. If these successive dampenings be unequal, some of the colors will not fall in their right places. Finally, if an oil painting upon canvas is to be reproduced, an additional impression is given from a plate upon which are embossed lines representing the threads of the canvas. The print is then varnished or glazed, like an oil painting. Many " chromos " thus produced can hardly be distinguished from the original pictures from which they are copied. Chromo- lithography has been brought to great perfec- tion in London, Paris, and Vienna. In the United States Mr. Louis Prang of Boston has within a few years executed many works not excelled by any produced in Europe. It is said that the first successful attempts to pro- duce chromo-lithographic portraits, in which the effects of the painter were closely imitated, was made about 1860 by Mr. E. 0. Middleton of Cincinnati, Ohio. He produced previous to 1866 a series entitled " Middleton's National Oil Portraits," several of which are admirable spe- cimens of the art. PHOTO -LITHOGRAPHY is the art of producing lithographic drawings by the action of light. About 1813 Joseph Nice"phore Niepce began his experiments for the pro- duction of permanent photographic pictures. These are the earliest on record, though not published till many years afterward. He used asphaltum as the substance sensitive to light, dissolving it in essential oil of lavender, and applying it as a thin varnish to metallic plates. After long exposure in the camera or under a clich6 of some sort, the asphaltum became in- soluble in the parts affected by light, and the picture was developed by dissolving away the unaffected portions. Undoubtedly Niepce had in view not only the production of a picture, but also the subsequent etching of the surface supporting his photograph, so as to yield en- graved plates from which copies could be printed in the press. In 1839 his method was superseded by that of Daguerre, with whom he had associated himself ; but his discovery of the sensitiveness of asphaltum formed the basis of the first photo-lithographic process deserving the name. This invention was made by Lemercier, Barreswill, and Lerebours of Paris, and patented in France in 1852. They proceeded by making a solution of asphaltum in ether, coating a clean lithographic stone with this varnish, and exposing under a nega- tive the dried surface so prepared. When the light had sufficiently acted through the trans- parent parts of the latter, the stone was washed with ether in abundance, whereby the un- changed and still soluble portions of the coat- ing were removed, and the stone was gummed, rolled up with ink, and etched in the manner practised by lithographers. This invention was based essentially on the discovery that the altered mineral pitch had an affinity for the greasy ink on the lithographer's roller, and this was applied to the production of designs on stone. The process gave crude results ; but it was the first of a series of six photo-litho- graphic methods, each of which must be re- garded as typical, and therefore worthy of description in the present article. M. Poitevin of Paris patented in December, 1855, a pro- cess for producing printable designs on stone. This was to a certain extent based on Mungo Ponton's original and fundamental discovery in 1839, that a sheet of paper sensitized with bichromate of potash applied in solution, and dried in the dark, was acted upon by light, giving a negative brownish picture, which, after exposure under an intercepting screen, resisted the solvent action of water. Becquerel had subsequently shown that the sizing of the paper played an important part in this phenomenon ; but Poitevin in 1855 was the first to discover and utilize the remarkable property possessed by organic matter altered by the action of light in the presence of bichromate of potash, which forms the foundation of most of the photo- mechanical printing processes now in use ; namely, that of repelling water and attracting greasy bodies, such as lithographic ink. To accomplish his purpose he coated the litho- graphic stone with a solution of bichromate of potash and albumen. After exposure under a negative, he applied moisture and ink followed by etching, in the manner practised by lithog- raphers, thereby obtaining an inky picture on the parts exposed to light. This process is in- teresting, although the results were far from satisfactory, inasmuch as the printing took place, not from a picture on the stone itself, but separated from it by a film of altered or- ganic matter; a circumstance which was not recognized by Poitevin, but was made use of many years later by Tessie du Motay, Albert,
