On the Sensory Discrimination of Intensities.

Although the relationship between sensory intensity and physical intensity of the excitatory agency cannot be directly established, it is possible to measure properties' of the process whereby sensorial discriminations of differences of intensity are achieved. This is the classical problem of psychophysics. -It is also a type problem for experimental biology. In general, as in the case of sensory intensity, the nature of the event or process which controls and determines a particular sort of regularity in the performance of an organism is not open to direct examination; it must be inferred from its quantitative properties. Inferences of this sort are likely to be based upon the use of averages of measurements of performance. An examination of the properties of the data of intensity discrimination indicates that such use of averages may be misleading, significantly in the sense that it involves ignoring a large part of the really useful information the measurements give. Averaging is commonly employed as a means of overcoming variation in the individual measurements. This variation, however, in homogeneous series of measurements, is a definite property of the reacting or performing organism, and cannot be dismissed as "experimental error"; its laws are discoverable, and describe an essential aspect of the process by virtue of which the measurements arise.1 A homogeneous series of measurements is one involving equal numbers of measurements on each organism at various values of the inciting variable, based upon the performance of either (a) a single individual, within a period in which the absence of irreversible spontaneous or other changes in capacity for performance is demonstrable; or (b) a group of individuals genetically alike and otherwise demonstrated to be effectively equivalent for the purpose of the measurements.2'1