Scientific American 7, 11.2.1860
L'Invention, for December, 1859, has an acoount of an improvement in the art of printing calico, paper-hangings, and maps as well as pictures, and which it pronounces of great importance in consequence of the cheapness of the process.
Our readers are aware that very great improvements have been made within a few years in the art of printing in colors by the lithographic process, chromo-lithography as it is called. One firm of lithographers in London have distinguished themselves especially by their improvements in this art. Oil colors are used, and we have seen some of their prints which it was really difficult to distinguish from oil paintings. The process consists in employing a stone of the full size of the print for each color and each shade. To place the paper with great precision on the several stones in succession, small pin points, secured firmly in the stone, project a short distance above its surface and pass through small holes in the paper. It is sair that more than 30 stones were used in re-producing Church's painting of Niagara Falls. The great expense of this process adapts it only to high-priced prints.
The object of the improvement of which we find an account in L'Invention, is to lessen the expense of this process, and the mode in which it is sought to be done is to produce all the colors and shades desired by combinations of the three primitive colors - red, yellow and blue. Our readers are aware that all the colors of the rainbow may be produced by combining these three colors. When a ray of white light passes through a triangular prism of glass, the seven colored rays of which it is composed are bent from their straight course, and as they are bent unequally, they are thus separated from each other. The color which is bent least is the red, and the colors are seen on a screen in this relation: -violet, indigo, blue, green, yellow, red. Now it is found that the red and yellow combined will produce orange, which, it will be seen, is between them in the spectrum. Yellow and blue will produce green, and blue and red will produce indigo and violet. Red, yellow and blue are consequentrly called the primitive colors; the French call them the mother colors. The Messrs. Acril, the inventors of the process of which we have spoken, avail themselves of this prolific property of the primitive colors, to reduce the number of stones, or rather of printing plates. This plan was tried many years ago in application to the printing of line-engravings, but owing to the fact that the sheets of paper must be moistened for printing in line, in order that the paper may be soft enough to be pressed into the channels in the metallic plate, it was found impossible to place them with sufficient accuracy on the successive plates. It is consequently necessary, in order to print several colors in succession, upon one sheet of paper to keep the paper dry, and in order to do this, the Messrs. Avril hit upon the plan of reversin the process of line-engraving - printing from the raised surface and producing the lights from depression as in wood-cuts.
The plan, then is to engrave a copper plate by cutting away the parts intended for the lights in the picture, leaving the parts for the colors raised - to have three of these plates, one for the red, one for the yellow, and one for the blue - and to so arrange these colors that they will overlie each other in a proper manner to produce the various shades and colors desired.