To Frederic E. Ives of Philadelphia, Pa., U.S., is due the idea of the optical V which led to the present cross-line screen which he used in his halftone plates, the printing surface of which was a bichromated glue enamel coating.
To the brothers Louis F. and Max Levy, also of Philadelphia, must be accorded the perfected halftone screen (grating) which today is universally used and has not been excelled since 1888. The Levy brothers coated selected plates of high quality optical glass with an etching ground, in which parallel lines were cut by means of a ruling machine.
The ruled lines were etched with hydrofluoric acid and filled in with Canadian balsam. The lines appeared in sharp con trast. The two diagonally ruled glass plates were faced at an angle of 9o°, sealed and later bound together with aluminium frames.
One of the important contributions to the halftone process was the invention of the highlight process. F. J. M. Gerland pat ented such a method in 1883 and the Bassani Company a me chanical improvement in 1925. The outstanding difficulty in the early days of the process was its inability to meet the demands for printing in quantity, rapidity, and the danger of the ink filling up the spaces between dots. This required the use of highly coated, hard-surfaced and expensive paper. In the last twenty five years, however, the process has been adapted to almost any kind of specification. Newspaper and poster plates are made through screens as coarse as 4o lines to the inch and pulp paper periodicals carry illustrations, sharp and full of colour, printed from plates up to no lines to the inch.
Colour Plates.—Photo-engraved colour plates are made to represent the subjects in the colours of the original colour photo graphs or coloured designs or from objects in their natural col ours.
The early attempts to reproduce colour effects were the man ual painting in of the different tints on black and white prints. Later stencils were used, but seldom for superimposed and colour combinations. The simplest form of colour reproduction followed the manual process and used photo-engraved key plates and dif ferent plates for each colour or shade. Later shadings and over lapping plates were used to achieve colour nuances and combina tions by overprinting.
The three- and four-colour processes used in 1940 are based on a Latin Treatise of 1611 by Dominis who described the additive system of complementary colours. Isaac Newton described light in its colour spectrum (1666). The German Le Blon in 1722, living in Paris, was the first to use three-colour printing with red, yellow and blue printing plates. Clerk Maxwell. the English physicist, was the first to publish (1861) the theory of colour reproduction by means of three-colour light filters. Ducos du Hauron and Charles Cros, independently of each other, published in 1868 their three-colour processes. The discovery (1873) by H. W. Vogel of optical sensitization led to a great advance in the reproduction of colour copy in monochrome by the use of ortho chromatic plates.
The brothers Lumiere of Lyon, France, gave the craft the Autochrome which with other colour records by similar processes often serve as a colour guide when the original painting is not obtainable. Modern panchromatic plates are the result of the suc cessive work of Vogel, Konig, Homolka, Eder, Miethe, Mees, Traube, Lehman, and other modern workers in the field of optical sensitizers. Colour separation negatives are made today on colour-sensitized gelatino-bromide dry plates or on colour-sensi tized collodion emulsion. The 'old-fashioned hand-press in the engraver's shop is now replaced by power presses. Colour reigns and is in steady ascendancy.