ABSTRACT
Plateau’s theory of the shape of the psychophysical function was described in chapter 1. It was based, in part, on empirical observations about artists’ productions of “midway” grays painted on disks. Plateau assumed that the artists used subjective equal ratio judgments in painting their grays. However, because he only asked the artists to find the “midway” gray, it is ambiguous from the perspective of direct measurement, whether a given artist used equal ratios or equal differences or some other interpretation of “midway.” As discussed at the end of Section 1.1, Plateau was aware of the potential ambiguity, for he wrote,
it seemed to me more rational, in order to explain the invariance of the general effect of the picture, to postulate a priori the constancy of the ratios and not the differences.
