Page:JOSA-Vol 06-06.djvu/45

 of a spectrophotometer. For the intercomparison of the color values of objects, without reference to the radiation by which they are viewed or the observer’s visual system, reflection and transmission curves are of great utility in colorimetrics, although such curves represent properties of objects rather than of immediate stimuli to color. However, in view of Hering’s principle of “the color-constancy of visual objects” (), representing the tendency of the visual processes to compensate for variations in spectral constitution and intensity of the illuminating source, these curves attain some direct significance for consciousness. Although colors due to selective reflection are of more common occurrence than those due to selective transmission, the latter are of greater scientific importance because of the far higher degree of selectivity which is obtainable by transmission than by reflection.

The spectral transmission distributions for a number of technically important materials are considered below.

A. .—There are several common applications of the principle of matching or of reproducing colors by the mixture of two or three stimuli, of constant relative spectral constitutions but varying proportions, which utilize selectively transmitting radiation “filters.” Although their transmissions may be varied within certain limits without deleterious effects, it is desirable to specify the transmission curves of certain of these filters which have been found satisfactorily to fulfill their purposes.

(a) Trichromatic Analyzer Filters. gives the transmissions of three filters employed in the Ives colorimeter ('; '), which is employed for the designation of the colors of materials in terms of three mixed elementaries, determined by these filters.

(b) 'Photographic Taking Filters. records the transmissions of four filters designed for making color separation photographic negatives on panchromatic emulsions. Rigid standardization of filters for use in this connection is not possible on account of the variations in sensitiveness to radiation of different wave-lengths exhibited by these emulsions, but the filters specified in the Table have been found to give fairly satis