a white of the same brilliance the critical flicker frequency is a function of saturation alone, so that colors possessing the same critical frequency will be of equal saturations. Since the spectral colors have radically different frequencies, with a minimum in the yellow at 575 mμ, it is necessary to add white to all except the latter to attain this equality. The per cents. of white (in terms of the total mixture) which were found necessary for the writer’s right eye are shown in Table 5.
Table 5
| Wave-length, mμ | 419 | 438 | 458 | 479 | 497 | 517 | 537 | 556 |
| Percent. white | 53.0 | 75.1 | 63.5 | 51.2 | 49.0 | 30.0 | 28.9 | 19.0 |
| Wave-length, mμ | 575 | 595 | 614 | 634 | 653 | 673 | 692 | . . . |
| Percent. white | 0.0 | 19.3 | 38.8 | 31.5 | 46.7 | 50.7 | 49.4 | . . . |
These figures are only tentative, as the sources of error in the experiment seem quite numerous. However, they indicate quite clearly that the differences in saturation of the spectral colors are of first order importance, so that it is highly improbable that the number of saturation steps from white is the same for them all. These conclusions appear to be corroborated by the relations of the spectral colors to white as represented on the color-mixture triangle (vide infra).
4. Stimuli for the Psychological Primaries
Westphal (98) has determined stimuli, as nearly as possible homogeneous, for arousing the psychological primaries in the average normal observer. They are: for red, extreme visible long-wave end of the spectrum plus a small amount of blue or violet; for yellow, 574.5 mμ; for green, 505.5 mμ; and for blue, 478.5 mμ. It is interesting to note that three of these stimuli (for red, green and blue) correspond quite closely with the three fundamental physiological primaries determined by König and Dieterici.
5. The Color Excitation Functions
