1. A method of experimentation is described which enables one to record objectively and quantitatively the discrimination by Mya between two intensities of illumination to which it is successively exposed. The indicator for this discrimination is a response at a given reaction time. 2. From the data so obtained it is found that the difference, ΔI, between the two intensities bears no constant relation to the initial intensity, I. Instead, the ratio See PDF for Equation varies in a consistent manner with I. As the latter increases, the ratio decreases to a certain point, after which it increases. 3. The data are analyzed in terms of the photochemical mechanism previously proposed for the sensitivity of Mya to light. It is shown that for the animal to discriminate by means of a given reaction between one intensity and another, the transition from one to the other must be accompanied by the decomposition of a constant amount of photosensitive substance in the sense organ. 4. A mathematical treatment of the behavior of the photochemical mechanism shows not only that the ratio See PDF for Equation cannot be constant as required by the Weber-Fechner law, but that it must vary in the way in which it does. The behavior of Mya under these conditions, therefore, supports the validity of the hypothetical physicochemical mechanism suggested for its sensitivity.