The influence of coloured surrounds and coloured backgrounds on visual thresholds

The present observations were a development of previous work on the effect of lights of different wave-lengths on each other when superimposed on the same retinal area (Roaf, 1928). The methods used were (a) to measure the threshold of a test light when shining alongside a modifying light, and (6) to measure the threshold when the modifying light shone on the same part of the retina as the test light, thus forming a background for it. It was assumed in that paper that by measuring the threshold of the test light when shining alongside the modifying light the observer’s eye would be in the same state of dark adaptation as when the two lights were superimposed, but it was pointed out that adaptation might be local and that “scattering” of light might have affected the results (Roaf, 1928, p. 254).