Page:Popular Science Monthly Volume 73.djvu/145

Rh existence. Of course, a developed psychology would endeavor to extend this plan to the entire range of consciousness. Fechner, however, confines himself to a single fundamental point—the relation between stimulus and sensation as generalized in Weber's law; although, just as Lotze before him, he considers other questions in a most suggestive manner, such, notably, as the seat of the soul, sleep and reminiscence. In pursuance of his early conviction,' that soul and body are but opposite sides of an identical existence (conscious), he took it for granted that their reciprocal action would be proportional. But this was belief, not science. Weber's work led Fechner to test the hypothesis, that the increase of physiological excitation holds the key to psychological changes. And his interest was stimulated by the fact that, if this could be proved accurately, his philosophy would benefit by so much indubitable evidence. Consequently, he was moved to verify Weber's law by numerous experiments, chiefly of a physical sort. Sensations of pressure and muscular effort, detected by the use of weights; sensations of temperature, determined by cold and hot water; sensations of light, handled by the photometer; and sensations of sound, observed by reference to falling bodies, all tended to confirm the same general relation between stimulus and the psychological event. Given what Herbart called a "threshold of sensation," and having fixed this as a constant for each class of sensation, Fechner found it possible to infer, by strict induction, that the intensity of the sensation is equal to the proportion of the stimulus, multiplied by the logarithm of the excitation, divided by the threshold of stimulus. In other words, we can obtain a formula for the quantitative relation of physical and psychological events considered as magnitudes. This formula, which provides a means of measurement, declares that the sensations increase proportionally to the logarithm of the stimulus. As a law, Fechner affirms dogmatically that it applies for internal (psychological) states and, within limits, reasons for which can be given, for external (physiological or physical) conditions. The result was obtained by three methods. (1) The Method of Differences which are Just Observable. This means that the operator finds, first, the least greater or the least smaller stimulus which can just be sensed as different by the subject; and then proceeds to add increments to this, or, inversely to subtract increments from it, till the intensity or diminution come into clear recognition. Divide the sum of the initial and the altered stimulus and you arrive at the differential of sensibility. (2) The Method of True (Eight) and False (Wrong) Cases. Here the operator applies two stimuli, which differ slightly, to the subject, and inquires whether the first is greater