Before carrying out this test, however, choice must be made of the trichromatic coefficients, and . According to Fig. 4 there is considerable doubt whether I.C.I. Illuminant should be taken to represent Munsell neutral gray; should we perhaps instead use the point given by disk mixture of the five principal hues in equal proportions?
It is believed that a definite answer to this question is given by computing the complements of R 5/5, Y 5/5, G 5/5, B 5/5, and P 5/5 by expressions analogous to Eqs. [7] derived from Eqs. [5], first with I.C.I. Illuminant C[1] and second with values obtained by equal-area disk mixture of the 5/5 samples.[2] As already noted, this disk-mixture point is designated by the symbol, DM. In the first case an erratic outline is obtained, in the second case a regular elliptical outline is suggested, as shown in Fig. 4 and in that part of Eq. [6] marked 6700°K. In this second case the complementaries fall between the adjacent primaries at regular distances from the neutral (DM) point. From whatever cause, it is apparent that the ten principal and complementary hues space symmetrically about the DM point far better than they do about the point C.
In Table IV are given the tristimulus specifications, the trichromatic coefficients, the dominant wave-lengths, and the colorimetric and excitation purities, for the five principal 5/5 samples (1919 data) on which this psychophysical system is based. Corresponding data are also given for the neutral (DM) 5/0 sample resulting from equal-area disk mixture of these five principal samples and for the five 5/5 complementary samples computed relative to the DM point by Eqs. [5]. From these data in Table IV may be computed for each of the ten hues, the tristimulus specifications, the trichromatic coefficients, and the colorimetric and excitation purities for any other sample of desired V and C. Values of dominant wavelength remain unchanged for each hue. The values of X, Y, Z, x, and y given in Table III of the Tyler-Hardy paper (11) were computed in this way by using Eqs. [5], [6] and [7].
Before proceeding to compare the 1919 and 1926 data with the data resulting from the psychophysical system thus developed, the meaning of the shift of the neutral point from I.C.I. Illuminant C to the DM point may be considered. Three possibilities may be noted.
First, perhaps I.C.I. Illuminant C is not as blue as that used in building up the Munsell system; perhaps clear north sky was used in selecting the Atlas papers. This possibility has been explored by computing the trichromatic coefficients for the five principal hues and their complementaries, both for the spectral distribution for Planckian 8000°K and for a blue-sky distribution (one of those used by the authors (13)) having a color temperature of approximately 25,000°K. The result is shown in Fig. 6 which also shows a similar plot for I.C.I. Illuminant B (4800°K). None of these illuminants renders less irregular the shape of the area delimited by the five principal 5/5 colors and
