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 stimuli must be mixed to match the homogeneous stimuli of the spectrum. (; 4, 15, 223-247).

(a) Any set of component stimuli, thus applied, may be regarded as physiological primaries, but when they are so chosen for any or all visual color systems as to account for the maximum number of facts they may be called the fundamental physiological primaries. Such primaries are stimuli, not colors; although they may properly be said to “have” a color. They are of the additive type because the stimuli are added to produce the required effects.

(b) The corresponding subtractive, or “pigment,” primaries are determined roughly by the spectrophotometric complementaries of the additive primaries. However, they do not consist of radiant energy but of absorbing mechanisms of one sort or another. In general they absorb from a “gray light’ spectral distribution—of a certain intensity—(vide infra) portions which are as nearly as possible equivalent in color mixture value to the respective, additive primaries.

C.. Another psychophysical function of the second type is the so-called visibility curve, which expresses reciprocally the intensities of radiant energy of different frequencies (or wave-lengths) which are required to match a standard in brilliance alone. The most recent average visibility values are given in.

(a) The conception of light, which is fundamental to photometry, is a psychophysical quantity defined as the product of the absolute power and visibility measures for any given sample of radiant energy.

(b) Relative light quantities are called luminosities.

D. are stimuli which when mixed additively in certain required proportions evoke a gray.

(a) Complementary colors are the colors evoked by these stimuli, in the given proportions, separately.

E. The definition of color as a strictly psychological entity does not preclude the legitimate use of such convenient expressions