length of the given stimulus. (c) Determine separately the areas of the three curves thus found. This latter operation can be performed by applying a planimeter to a graph of the resultant curves or—with sufficient accuracy—by finding the sum of representative ordinates of each curve, taken separately at uniform, small, intervals—such as 10 mμ—throughout the range of the curve in question. (4) Reduce the three areal values thus obtained to percentage form, so that their determined ratio remains unchanged but their sum becomes equal to 100. The color excitation values can now be expressed by means of two numbers, representing the red and violet excitation percentages, that for the green being obtainable by subtracting the sum of these two values from 100.
As already pointed out, it is in general impossible to reverse the above process, and to convert color excitation specifications into definite spectrophotometric form, because there are an infinite number of spectrophotometric conditions for the majority of color excitation ratios. However, it is possible by means of the color triangle to determine stimuli for given sets of excitation values. The most feasible method of procedure is to plot the position of the given color in the triangle and to note its relation to the locus of the spectral colors. If it lies outside of the area bounded—on two sides—by this locus it possesses no realizable stimulus. If it lies exactly on the locus, in a region of the latter which exhibits curvature, it possesses a unique condition, viz., the homogeneous spectral stimulus having a wave-length indicated by its position with respect to the wave-length scale plotted on the spectral locus. If it falls on a straight portion of the locus in question, it can be evoked by the homogeneous wave-length which immediately corresponds with its position, or by mixtures of any stimuli having wave-lengths represented on either side of it in the given straight portion of the locus, the proportions of these mixed stimuli being determined by the “center of gravity” principle (vide supra). If the point representing the given color lies within the area bounded by the spectral locus, the color can be produced by mixtures of spectral stimuli lying at the intersections, with the locus, of any straight line passing through the