Abstract
The colour difference required for the threshold is related to the statistical fluctuations in the rates of absorption of quanta by the 3 colour components in the visual system. A revision of the Thomson-Wright curves has been used as an estimate of the 3 cone sensitivity curves. A derivation of the shape of these curves is given in an appendix. A transformation from the Young-Helmholtz scheme into the Hering scheme is introduced to account for the independent variation of red-green and yellow blue discrimination with visual angle. Furthermore it is assumed that with increasing luminance the blue system has to be given an increasing weight in hue discrimination.