[Pupillometric studies of the fundamental psychophysical law].

Formulating the correlations bearing both their names, Fecher and Stevens, used aside form Weber's law delta I/I0 = const. hypothetical assumptions about the sensitivity increase at threshold (delta E = const. and delta E/E = const., respectively). closer investigation of the visual system shows that not only these hypotheses will have to be discussed but that Weber's law, too, has to be interpreted more precisely. Measurements of the pupillomotor and sensory threshold yielded delta I/Io alpha = const. with alpha approximately equal to 0.7, consistent with the results of other authors. Irritation with the degree of stimulation, as measured by the variation of the pupillar radius at the dark-adapted eye, is well describable by the power function E = k x In with n approximately equal to 0.35. It is shown that this form is the integrated from of the "generalized Weber's law", where n approximately equal to 1--alpha holds if Fechner's assumption delta E = const. at threshold is true. From the results of Stevens ( n approximately equal to 0.33) and the values of alpha for the visual system. For the other systems, e.g. that of length estimation, the assumption is wrong. To enable uniform description of the varying situation, the more general assumption delta E/E gamma = const. is proposed for the sensitivity increase at threshold; with this approach n = (1--alpha)/(1--gamma) (alpha not equal to 1, gamma not equal to 1) holds.