The usual interpretation of this phenomenon, namely, that it consists in a subjective ‘contrast’ impression of the eye – a red surface in red light looking brighter, a green surface darker, than its surroundings, and thereby causing the illusion of white or black – is a typical onlooker-interpretation against which there stands the evidence of unprejudiced observation. The reality of the ‘white’ and the ‘black’ seen in such cases is so striking that a person who has not seen the colours of the objects in ordinary light can hardly be persuaded to believe that they are not ‘really’ white or black. The fact is that the white and the black that are seen under these conditions are just as real as ‘ordinary’ white and black. When in either instance the eye registers ‘white’ it registers exactly the same event, namely, the total reflexion of the light by the surface struck by it. Again, when the eye registers ‘black’ in both cases it registers an identical process, namely, total absorption of the light.¹