The Colour Change of Monochromatic Light with Retinal Angle of Incidence

Abstract

The few previous measurements of the small change of apparent colour of a monochromatic light stimulus when its angle of incidence on the foveal retina is varied (or SC II effect), have been confined to changes of apparent hue. A differential, three-colour mixture method, with alternating instead of juxtaposed presentation of the test and comparison stimuli, has now been applied to determine the complete colour change, hue and saturation. For the main subject, the results show hue shifts of the expected kind and a small but significant supersaturation of the apparent colour of an obliquely, compared with a normally incident stimulus, in the wavelength range 485 to 510 mµ. From earlier qualitative observations, such a supersaturation appears to occur for most subjects. The SC II colour changes are attributable to differences in the directional response of the receptors associated respectively with the three colour systems of the trichromatic scheme. From the relative spectral sensitivities of these systems and the complete SC II colour changes, the variation with wavelength of the directional sensitivity of each system has been computed for the data of the main subject. The possibility of explaining the derived wavelength variation in terms of self-screening by visual pigment present in the receptors in sufficiently high density, is examined. Self-screening appears not to be a factor for the blue-sensitive system, and although for the red- and green-sensitive systems it predicts minima in the directional sensitivity v. wavelength curve, resembling those observed, the quantitative agreement is poor and there are other objections. The wave-mode disposition of light in the receptors, now being studied, should yield a better explanation.