exposure continued in use, though the application of a crosslined screen with uninterrupted exposure readily suggested itself. I got practical results with such screens, composed of wire bolting cloth, as early as 1883, but the possibility of obtaining varied halftone effects with the single-line screen by varying the proportions of the divided exposure, coupled with the difficulty of getting a sufficiently perfect cross-line screen of any size, kept the singleline screens in use. These were generally produced by photographing a sheet of parallel lines printed from a steel plate or a lithographic stone ruled by machine, reducing the size to get closer rulings. But the clouding of the open lines through halation, which left those lines most transparent in the middle and gradually less so towards their edges, though apparently a desirable feature, proved in practice to be a hindrance, and even where the lines were cleared as much as possible these photographed screens kept out so much light as to necessitate a considerably prolonged exposure, and when doubled to make a cross-line screen the difficulty was correspondingly increased. The way forward was manifestly through a proper ruling on glass. To get this crosswise and to overcome the diffraction effect of surface rulings I conceived the idea of etching the ruled lines with hydrofluoric acid and blackening the depressions, which would give a theoretically perfect screen and a correspondingly perfect halftone reproduction. Small screens of this kind were put to use in 1887, and with the collaboration of my brother Max in this direction the idea was finally realized in full and patented by us in 1891.

Following these successive improvements of the halftone process, its uses and applications advanced by leaps and bounds. As early as 1890, in an article on this subject in the American Annual of Photography for that year, I found occasion to remark as follows:

“The influence of the halftone process in the dissemination of intelligence is already making itself felt in the pages of standard and current publications everywhere, and its value as an adjunct in raising the standard of art culture and of artistic appreciation is commensurate with its almost limitless possibilities. It is the graphic method of the future, combining as it does, in a higher degree than any other, the illustrative and the æsthetic elements of art."

During the quarter century which has since elapsed two further advances of striking importance have been made in the application of halftone photo-engraving; namely, the three-color printing process and that of rotary photogravure.

The production of pictures in colors had become the special province of lithography, although for many purposes the greater rapidity and corresponding economy of printing from relief plates made this latter method preferable. In either case a considerable number of successive printings, in different colors from as many different forms, is necessary, and printers and lithographers working with colors have sought to reduce the number as far as possible. To some extent this could be done by printing one color over another to produce a third, and, by gradations of tone in each, bring about various tints of the combined colors. By combining in this way the three principal colors, red, yellow, and blue, a wide range of other colors and tints can be produced, the effect depending on the degree of skill with which the engraver or lithographic draughtsman works up the three different printing forms which make up the combination.

A very important advance in the production of color effects by photo-lithography is that known as the Polychrome process, patented in Europe by Henri J. Burger of Zurich, in 1897. In this process the printing forms, instead of being worked out by the lithographic draughtsman, are produced photographically, the negative of the subject, with all but the desired features stopped out, being used for one or two of the color forms, and transparent positives from the same negative, with the correlative features stopped out, being used for the other colors. A key plate from the original negative completes the effect. An essential feature of the polychrome method is the use of the asphaltum process to produce the printing form on the stone, the developed film presenting a very fine grain, corresponding with the gradations of the negative or positive applied in the exposure, and through which the stone is etched in preparation for the printing. This method is capable of producing effects of a high artistic quality and is especially applicable for subjects of large dimensions.

With the development of the halftone process the way was open for the production of relief-plate engraving by the threecolor process of photography which had already previously been utilized, but only within the limitations of collotype and photo