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Supplementary Notes on Screen-Plates
575
cross-line screens at present marketed are. I believe, 400 lines to the inch, it would at once occur to a photo-engraver to ask how Mr. Powrie obtained his original negative screen-plate of sufficient fineness. The reply is that Mr. Powrie makes black screens exactly like his color screens, and simply goes on reducing the lines and spaces until he obtains the fineness he wishes. Pie informed me that he commenced with quite a coarse screen ruled in the usual way, but with great accuracy, and from it he has made with ease screens of a fineness equal to 1,600 lines to the inch of perfect uniformity. The diagram, Fig. 156, shows quite clearly how this can be done by exposing and developing the plate, blackening these lines by a ferro-tannic process, and then recoating and replacing the plate in a different position. The second printing will give a screen of twice the fineness of the original, which is now replaced by the manufactured screen, and this is made to serve its fineness, and so on. I have handled black-line screens, both fine and coarse, made in this way, that for all practical purposes would serve as well as the ruled cross-line screen does for making half-tone blocks, and their cost should be enormously cheaper if, as asserted, one man. by means of a suitable printing machine, can make
dozens of them a day. Of course, any angle and any pattern of line can be produced in this way, and any width of ruling, any relationship of white space to black line."
It is obvious that there is some similarity with the method outlined by du Hauron (see p. 454), and in later experiments Powrie also utilized the former's idea of placing the matrix at right angles to the original direction of the screen lines, thus obtaining one continuous line with interposed rectangles of the other two colors. As regards the printing of these plates a similar ruled plate was used with the lines at right angles to the negative; but after making the first exposure, the printing frame was turned through a slight angle, so that the image of the line was projected not on top of the first exposure, but adjacent to it, as shown in Fig. 157, and then the frame turned through a similar small angle in the opposite direction, so that again the line was projected on the other side of the original line. Each line was in this way broken up into three lines, overlapping its neighbors on each side, the effect of which was to remove the white in the positive.2