ratio in which the intensity is measured in energy terms, while photometric purity may be defined as a similar quantity based upon evaluations in terms of light units. Although the choice of the particular constituent with respect to which the purity is to be estimated is necessarily more or less arbitrary, we may define—as a special case of considerable importance—the colorimetric purity, which is the ratio in luminosity terms, between the dominant homogeneous constituent and the total sample, where the “dominant homogeneous constituent” comprises a range of wave-lengths not greater than that corresponding to a single chromaticity threshold in the given spectral region, and has a dominant hue identical with that of the total sample, the intensity of the homogeneous constituent being arbitrarily so adjusted with respect to the total intensity that it can be mixed with “gray light” in such proportions as to yield a color-match with the total sample. This last definition corresponds with that of “per cent. white” in the method of colorimetry by “monochromatic analysis,” but evidently involves psychophysical considerations in so essential a manner as to have very little physical significance.
F. Mode of Incidence—The color which is evoked by any adequate stimulus depends not only upon the spectral distribution of the latter, but also upon certain further conditions which may be called those of its mode of incidence. These conditions include: (1) the type of color system possessed by the observer, (2) the portion of the retinal field stimulated, (3) the size of the field, (4) the momentary state of adaptation of the optic nervous mechanism, and (5) the excitation processes in adjacent visual areas. In accurate work these factors require specification. In general, we assume pure cone vision of the normal trichromatic system, central fixation, a size not exceeding three degrees, and a gray contrast field of the same apparent brightness as the given stimulus light. (vide infra, for conditions of pure cone vision.)
3. Psychophysical Terms
This section deals with the terminology of the relation between color and its stimulus. The study of this relation constitutes the science of color sensation.
