The set of ellipses that represent difference thresholds at constant luminance in the chromaticity diagram can be readily explained by the Young-Helmholtz theory if one assumes Fick＇s hypothesis (that dichromacy arises from trichromacy by fusion of two fundamentals). We find that the blue fundamental functions as an independent receptor of a generalized Fechnerian type, while the red and green fundamentals remain on the contrary closely linked. The existence of the ellipses is interpreted, on the other hand, by means of a simple statistical theory, taking into account Crozier＇s Law.