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 For normal vision these primaries are: black, white, red, yellow, green, and blue. (', 251-252; ', 21). Red and yellow may be grouped together under the designation of “warm,” while green and blue may be classified under the designation of “cold” primary chromas.

E. .—The three attributes of color can be treated as quantities and specified numerically, if all discriminable colors are conceived to be arranged into a system such that neighboring members differ from one another in each of the three attributes by just noticeable degrees (or threshold steps). (', 207-215; ', 1-10). Such a system (vide infra) is necessarily three-dimensional (, 18-31), and three ordinal values, representing the positions of a given color in the several dimensions are needed to define the color. The spectral chroma scale, considered more in detail below, is an application of this principle of color measurement to the study of the dependency of chromaticity upon wave-length.

A. .—The adequate stimulus of color consists of radiant energy of certain frequencies or wave-lengths which have various stimulus values depending on the type of visual response system under consideration. The term “radiation” is often employed as a brief equivalent of “radiant energy,” although this usage tends to confuse the process of radiation with the outcome of the process.

B. is an arrangement of radiant energies in order of their respective frequencies or wave-lengths. It should not be confused with the color spectrum which is a series of colors aroused by part of the physical spectrum.

C.. The properties inherent in any sample of radiant energy which determine its capacity as a color stimulus are completely specified by its spectral distribution, which expresses the “intensity” for any frequency (or wave-length) as a function of the frequency (or wave-length) in question.