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 we complete our nomenclature. We cannot use B for black because B has already been used for blue, and therefore we use N. for niger the Latin word for black.

Briefly stated then this system of color instruction is comprised under the five general heads : Spectrum Standards; Pigmentary Standards based on the spectrum standards; Maxwell Rotating Disks in the pigmentary standards and Black and White ; a Color Nomenclature based on the accepted standards and their disk combinations ; and colored papers made in accordance with these standards and their combinations.

For spectrum standards, six definite locations expressing the natural aesthetic or psychological impressions of red, orange, yelllow, green, blue and violet are selected. Six standards are chosen instead of a larger number as for example twelve, because for the purposes of a nomenclature the smaller number is more convenient than a greater number. The six are selected rather than three, four or five, because while in the consideration of colored light alone the smaller number would possibly suffice to form by combinations imitations of all other colors, any number smaller than six is entirely inadequate to form by pigmentary or disk combinations fairly good expressions of the corresponding spectrum color combinations.

In selecting the spectrum standards special prominence has been given to the psychological color perceptions of experts in determining those locations in the spectrum best expressing the color feeling of red, orange, yellow, green, blue and violet, while the purely scientific consideration of these several questions has not been ignored or lightly treated.

For pigmentary standards the best possible pigmentary imitations of the six spectrum standards are secured and to these are added the nearest approach to white and black that can be produced in pigments.

Pigmentary standards on which to base a nomenclature are valueless without some means by which measurements of stand-