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458 History of Three-Color Photography
basing his statements on Konig's curves, after rejecting Clerk Maxwell's, suggested that a color a little to the right of the E line, about wave-length 5500, was the best for the green sensation; and for the red about DJC, which would be about 6000, and that for the blue about 4550. These were the dominant wave-lengths for the taking screens ; other colors were chosen for the viewing screens (see p. 475).
C. Kaiserling11 stated that the transmissions of the Joly negative screens were for the red, from 6700 to 5500 with maximum at 6000 to 5800; for the green, from 5700 to 4700 with maximum at 5300 to 5150; for the blue, 5200 to 4300 with maximum at 4800 to 4600.
Mees12 pointed out that as in this process the negatives are frequently reversed and viewed as positives, a compromise between the taking and viewing filters must be made, and, therefore, the filter zones should touch but not overlap. The reason for this is that the eye is extremely sensitive to changes of color, or wave-length, where the overlap of the filters should occur, as stated by P. G. Nutting13 and it is important, therefore, that there should be no great gaps between the transmissions of the filters.
Mees and Pledge1* stated : "But if it is of importance that the taking filters should overlap, or at least touch, it is of equal importance that the viewing filters for the additive process should be as pure as possible, especially if the taking filters already show an overlap. Consequently the best practical compromise will probably be three filters transmitting red from 5900 upwards ; green, from 5900 to 4900 ; blue, from 5000 downwards. It may be remarked that the higher the lower limit of the red filter is, the better will be the color of the pure reds, and the greater the difficulty of distinguishing between yellow-greens and pure greens. Similarly if the lower limit of the green filter is too far into the blue, there will be difficulty in obtaining bright yellows, if not far enough, blue-greens will be too blue."
That white should be rendered as white and without color, it is important that the screen-plate shall be of a neutral tint, or free from color, which Mees described as the "first black condition," and ascribed it to McDonough15 but as will be seen from page 454 this requirement was fully recognized by Ducos du Hauron. To fulfill this condition, the area of the units must be altered, and not their depth of color, the latter being determined by the necessary filter cuts.
The size of the screen elements is determined by their invisibility, the thickness of the medium necessary to obtain sufficient depth of color, and the irradiation occurring in the emulsion film.
The Pattern of the Screen Elements. — In the manufacture of screenplates, it is obvious that the pattern of the elements plays no unimportant part, and this subject was very fully treated by Welborne Piper.16 In making the screen-plates by methods which involve the application of dyes to a film, trouble may be caused by the overlapping of the margins, or the