Page:Journal of the Optical Society of America, volume 30, number 12.pdf/35



Probably the most satisfactory way by which the data reported in this paper can be tested psychologically is to compare them with the data obtained by the Newhall subcommittee. These data, some of which are reported in the final paper of this series by Newhall (15), have been based upon observations made on samples of the Book of Color to see what changes, both in magnitude and direction, would be necessary to bring about a still better representation of the ideal psychological color system, in which the steps are truly representative of equally perceptible changes in any single color attribute.

Regarding value, we already know that a definite relationship between value and reflectance was intended by Munsell, and that the Munsell value scale was intended to represent equal sensation steps. In regard to hue, we know, from, that the dominant wave-lengths of certain hues tend to change in accord with the requirements of a psychological system when the illuminant point is taken for a reference point. In regard to chroma, we can compare the excitation purities for samples of the 70 Atlas colors measured in 1926, both with excitation purities obtained from the data of the psychophysical relation developed in an earlier portion of this paper, and with excitation purities obtained on a basis of the Glenn-Killian (16) data, modified by the Newhall (15) data. enables this comparison to be made.

As the comparison is made, we begin to realize that a psychophysical system such as developed in this paper and a psychological system may not be as far apart as might have been supposed. However, the 1926 data at the low Munsell values in most cases agree with the data indicated by the Newhall report better than with those indicated by the psychophysical system. At the other values the data of are not so conclusive, although, except for value 8/, the agreement of the 1926 data with the psychological