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32 History of Three-Color Photography
diaphragm e, e. Neutral wedges K for the pigment to be examined, and k, k, k for the standards are used to reduce the light. The eyepiece 1, focused for the diaphragm, serves for comparison of the two fields.
The filters for this instrument were made as follows: for the red or vermilion tartrazin 2 g., erythrosin 1 g. ; for the blue, blue carmin 1 g., acid rhodamin 2 g. ; for the green, tartrazin 1.2 g., blue carmin 1 g. ; these quantities being per square meter of area. As an example of the analysis of a pigment the following may be taken as typical : a light chamois paper was measured, and found to be matched by 19 per cent blue + 38 per cent green -f 47 per cent red. Then as equal parts of the three colors form white, this resolves itself into 19 white + 19 green + 28 per cent red. As equal quantities of red and green make yellow, the final reading becomes 19 white -(-19 yellow -f 9 per cent red. The amounts of the three colors being obtained by the shifting of the neutral wedges, which are provided with graduated racks and pinions.
L. Geisler51 suggested the use of the Maxwell color top for ascertaining the intensity of the printing inks, with the view of obtaining blacks and greys, without the predominance of one or the other color. Slotted cards were covered with the inks to be tested in full intensity, and then arranged on the color top, so that they showed equal angles. On rapidly revolving the same, it was at once seen if any color predominated, and the angles of the sectors could be altered till a perfectly neutral grey was obtained. If this could not be done, then the inks were to be rejected. As an example, let it be assumed that the angles of the sectors to obtain a neutral grey were respectively, yellow 120, red 140 and blue 100 degrees. It is at once clear that the yellow and blue are more saturated than the red, and the latter must be taken as the standard. To obtain equilibrium the intensity of the blue and yellow must be reduced by diluting the inks with varnish. Comparing the red, the least saturated, with the others, one can write :
actual intensity 140 deg. for blue =
for vellow
highest intensity 120 deg. actual intensity 140 deg.
highest intensity 120 deg.
One ought then to mix 100 parts of blue ink with 40 parts of varnish, and 120 parts of the yellow with 20 parts of varnish to obtain in each case 140 parts of tint, which should be used.
Clay's Researches. — One of the most important papers on the application of Maxwell's curves to subtractive color work was by R. S. Clay52 and his reasoning is so complete and detailed that no excuse is needed for dealing with it at some length, particularly as very little attention has been given to it.