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A. .—There are two important, general types of psychophysical functions which occur in colorimetrics, (1) a type which expresses a direct relation of dependency between a psychological color attribute (vide supra)—measured in threshold steps—and a stimulus variable, and (2) a type which formulates relations between two or more stimulus variables, such relations depending upon and expressing the conditions for the equation in one or more psychological dimensions, of the colors due to different stimuli.

B. .—An example of the second type of function appears in the color excitation curves for various visual color systems, which curves show in what proportions of intensity a number of selected color