Colour Receptors in the Human Fovea

Abstract

DR. F. W. EDRIDGE-GREEN suggests that, if Prof. Hartridge's observations in Nature of July 20 were correct, the stars should appear to change colour as their light falls upon different cones of the fovea. Prof. Hartridge, in reply, says "Dr. Edridge-Greeryis quite right. On either the three-colour theory of Thomas Young, or on its modern counterpart, the Wundt-Granit hypothesis, one would expect a point source, and therefore a star,to undergo subjective changes of colour as its image is caused to move over the retina. Some stars do change colour, a fact usually explained on purely physical grounds ; I have also noticed changes of colour of local sources ; under conditions where physical explanations did not appear to apply (Nature, July 20, p. 97). Why, it may be asked, are such colour variations so seldom seen when the eye is being used in normal vision ? The answer is not a simple one, since many factors play a part. In the first place the retinal image, even of a point source, falls on a considerable area of retina. This is partly due to diffraction, and partly due to aberrations. In consequence, many photo-receptors are usually stimulated at the same time. But further, colours which would normally be seen are eliminated by the anti-chromatic responses. In the third place, there is a process which tends to smooth out differences in response. This is now under further investigation, because it is hindering progress. When a method has been found of putting this process temporarily out of action, a new avenue of approach to the essential problem of vision will probably have been found."