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 are being made at the Bureau of Standards, and will be considered in later reports of the present Committee.

(c) , besides having been elaborately described by its author in a number of publications, has been given two concrete exemplifications. The most elaborate one, “,” presents approximately twenty-five hundred colors which are systematically indexed in order of their color tone and content of black and white respectively. An abridged system, “,” comprises seven hundred and sixty-eight of the principal colors used in the more comprehensive system. Ostwald’s arrangement, like that of Munsell and Ridgway, is based upon psychological rather than physical criteria. Kohlrausch () has made a spectrophotometric analysis of sixty of Ostwald’s colors taken from three of his color circles having different saturations or reflectivities. Kohlrausch has furthermore computed the excitation values of these colors on the basis of König and Dieterici’s excitation curves. He has also made direct monochromatic analyses of the colors in question.

It is not our purpose in the present Report to consider in detail the various methods which are in use, or which have been proposed, for the practical measurement of color. This important task, which involves the description of instruments and the establishment of a terminology for each of the methods, is reserved for a later report. However, it is desirable here at least to catalogue the available systems of colorimetry, and to consider in a preliminary way some of the problems which arise in connection with them, especially that of the reduction of data obtained by the various methods to a common comparison basis.

The color of an object, considered as an impression which the object produces on the observer, is determined by at least three general sets of factors: (1) the physical characteristics of the object, (2) the physical characteristics of the radiant energy falling upon or emitted by it, and (3) the nature and condition of the observer’s visual apparatus. Our control over color in its