ALL PRINCIPLE HUES MAY BE OBSERVED IN SCOTOPIC VISION

A test of the prediction was carried out by Stabell and Stabell (1965). The successive phases of the experiment were as follows: (1) Dark adaptation for 30 min. (2) Pre-stimulation extrafoveally for 30 s using one of several colour filters in front of the eye at an intensity 1 log unit above the specific-hue threshold measured for the filter used. (3) Dark adaptation for 30 s. (4) Test stimulation at scotopic intensity levels in the pre-stimulated area using a green monochromatic test light.

Pre-stimulation with a red colour filter produced a blue colour upon the scotopic test stimulation, a yellow pre-stimulation filter produced blue or blue-violet, a green filter, blue-violet or violet, while blue and blue-green filters did not produce any chromatic effect upon scotopic test stimulation.

The reason for the failure to produce red, yellow and green scotopic contrast colours soon became apparent in a follow-up study where it was found that by increasing the level of chromatic adaptation (i.e. increasing the time of pre-stimulation, reducing the time interval between pre- and test stimulation, and increasing the size of the pre- and test-fields) scotopic red, orange and green colours could be produced. Thus, pre-stimulation with a green-blue colour produced a red colour upon test stimulation at scotopic intensities, a blue colour produced orange, and a purple colour produced a blue-green colour.