Page:Letter Circular 454; hues of the spectrum colors (IA lettercircular45454nati).djvu/3

 ''3. Retinal Illumination.'' If the spectrum is very weak so that the retina receives only twilight illumination, its appearance will be the same to the normal as to the totally color blind observer. If the spectrum is very strong so as to be dazzling, it likewise exhibits no hue. Just weaker than the dazzle stage, it appears predominantly yellow and blue, and just stronger than the twilight stage, it appears predominantly red, green, and violet. At intermediate retinal illuminations, it takes on the characteristic appearance, but the appearance depends on the particular illumination.

4. Pre-exposure Stimulus. The spectrum looks different to a dark adapted eye than to a light adapted; to a red adapted eye than to a green, and so on.

In the work to be summarized, the observers were either presumed to be normal or found by test not to be seriously anomalous; they looked directly at the spectrum so that a region near the center of the retina was used; the spectrum was neither too weak nor too strong to prohibit the characteristic appearance, and the pre-exposure stimulus was darkness, sunlight, overcast-sky light, or some other stimulus which was presumed not to introduce chromatic fatigue. Within these limits various observing conditions were used. The lack of agreement is to be ascribed to the se variations, to individual differences between the observers, and to the usual experimental error which prevents an observer from duplicating his results exactly even though observing conditions are identical.

The following table gives the wave lengths corresponding to red, orange, yellow, green, blue, and violet, and their intermediates according to various authorities. The entries have been arranged chronologically. The average values given are equally-weighted arithmetical means. No average wave length for red is given because some of the authorities state that pure red is not to be found in the spectrum, spectrum red being reported as slightly orange. For the same reason no maximum value of wave length for red could be given, nor any average or minimum values for violet.

Bibliography.


 * H. v. Helmholtz, Handb. d. Physiol. Optik, p. 236 (1866).
 * W. v. Bezold, Die Farbenlehre, pp. 24-27 (1874).
 * J. C. Donders, Farbengleichungen, Arch. f. (Anat. u.) Physiol., 533-535 (1884).
 * O. N. Rood, Modern Chromatics, p. 26 (1890).
 * H. Voeste, Messende Versuche über die Qualitatsänderungen der Spectralfarben in Folge von Ermüdung der Netzhaut, Z. Psychol. u. Physiol. d. Sinnesorg., 18, 257 (1898).