The Fundamental Colour Sensations

IF pressure, sufficient to be thoroughly uncomfortable, is maintained by the fingers on the eyeball, numerous violet dots will eventually appear in among the other entoptical phenomena. Each dot lasts a second or so. If the pressure is suddenly released, all the entoptical phenomena disappear except the violet dots, which are left for a few seconds against a dark background. They are fairly uniformly distributed over the field of view, and may number anything up to, say, 100. From the fact that all the violet dots are of exactly the same hue, saturation and intensity and of approximately the same size, one is probably justified in assuming that they are the fundamental violet sensation. By looking at a spectrum with the other eye (light-adapted) it is possible to match their hue against a spectral hue. They match 440·5 mµ, the standard deviation of each observation being ± 5 mµ and that of the mean being ± 0·5 mµ. They are considerably more saturated in colour than spectral violet. Their apparent angular diameters are of the order of 10'. From the tritanopic isocolour chart given by Pitt 1 , which depends on a description by König of five pathological cases of tritanopia, one can deduce that the fundamental violet sensation is equivalent to the sensation produced by 435 mµ, except that it is more saturated. Considering the inadequacy of the data available to Pitt, the discrepancy between 440·5 mµ and 435 mµ is not significant.