Page:Journal of the Optical Society of America, volume 30, number 12.pdf/29

connecting the experimental data with the lines of dominant wave-length, designated by crosses in Fig. 4, were determined on the basis of the psychophysical system described below and are taken from Table III of the Tyler-Hardy paper.

Fig. 5. In this figure is shown the degree to which the dominant wave-length of each Munsell Atlas sample (1926 data) departs from the average dominant wave-length for all samples of that hue designation, as derived from Tables III and IV. In each case, the arrow shows the direction of the departure and its length indicates the proportional part of the dominant wave-length interval to the adjacent hue. It illustrates the relative constancy of the dominant wave-lengths of, for example, the yellow samples and the shift of dominant wave-length for the blue-green and red-purple samples from value 8/ to value 2/. These dominant wave-lengths are all derived with I.C.I. Illuminant C as the reference point; the results would in some cases be importantly different if DM (Fig. 4) were used as the reference point.

The 1926 data of Fig. 4 are plotted in different form in Fig. 5 in order to discover whether such variations as there may be in the relation between hue and dominant wave-length are random differences, or whether there may be some regularity in the pattern of variation. Each arrow point, at the value plotted, indicates the direction and magnitude of the difference between the dominant wave-length of that sample and the average dominant wave-length of all the samples of that hue designation, the length of arrow indicating the proportional part of the whole A interval. These average dominant wave-lengths were computed from Tables III and IV.

From an examination of Figs. 4 and 5 and reference to Tables II, III and IV, we may conclude that there are many deviations from dominant wave-length constancy among the samples for a single hue and its complementary. Some of the deviations are obvious from inspection of the samples, others are not. The hue which has the most constant dominant wave-length is yellow; the hue exhibiting the least constant dominant wave-length is blue-green, the deviation for this hue being nearly enough to extend to the average dominant wavelength of Munsell blue. In addition there is a very definite trend in some of the hues for the dominant wave-lengths of the samples of high Munsell value to be greater than those for the lower values. For example, with the blue-green samples, from values 8/ to 2/, dominant wavelength progresses from 506 through 500, 492, 496, 490 to 485 and 487 mμ. A reverse progression holds for the red series, the samples of high Munsell value being shorter in wave-length than those of low value, the progression being from a dominant wave-length of 599 mμ at value 8/ to 615 (or 640) mμ at value 2/. Such progressions are suggestive of the Bezold-Brücke phenomenon (12), the change in hue produced by an increase in luminance, but it must be recalled that the samples of each Munsell hue represented by Fig. 5 differ in purity as well as in luminous apparent reflectance. So the more or less regular departures from constant dominant wave-length shown in Fig. 5 for all but the yellow and green-yellow hues may exemplify the hue change by admixture of achromatic light (19) as well as the Bezold-Brücke phenomenon, and indeed the latter phenomenon would seem to be secondary for Munsell red and yellow-red because the