the generalization to which the name "Weber's Law," or the "FechnerWeber Law," or the "Psycho-physical Law" has been given. Referring to this discovery, in the preface to the first great book on physiological psychology, Fechner affirms: "The empirical law which forms the principal foundation, was laid down long ago by different students in different branches, and was expressed with comparative generality by E. H. Weber, whom I would call the father of psycho-physics." The law summarizes mathematically the relation between physiological stimulus and psychical sense-perception. It is based on the fact, familiar in common experience, and now authenticated by numerous observations and experiments, that the difference between two sensations bears no direct proportion to the actual difference between their stimuli. Granted that the least observable difference be a constant, then, the strength of sensations does not grow in proportion to stimulus, but much more slowly. Weber's experiments were directed towards measuring the exact proportions, and involved comparisons of lines by the eye, of weights and of tones. The resultant generalization has been formulated in various ways. The most direct are as follows: "In order that the intensity of a sensation may increase in arithmetical progression, the stimulus must increase in geometrical progression"; or, as put more briefly by Fechner, "the sensation increases as the logarithm of the stimulus"; or, as Delboeuf has it, "the smallest perceptible difference between two excitations of the same nature is always due to a real difference which grows proportionately to the excitations themselves." Like all laws, so-called, this one is an abstraction from experience. Consequently, it has been subjected to various interpretations, has been transformed and criticized, and even denied. Again, like all laws, so-called (e. g., Boyle's law), it holds good only within limits, and round this aspect of the matter multitudinous experiments cluster. Space forbids more than a reference to easily accessible literature. Whatever psychological experts may consider to be the present status of the conclusion, Weber's withers are unwrung. His crowning achievement was to have shown that measurements and mathematical methods can be applied in this region of experience. He thus served himself the founder of the Leipzig line, the torch passing from him to Lotze, to Fechner, and finally to Wundt.
As at the beginning of modern European thought, in Descartes, Spinoza, and Leibniz, so here in the Leipzig men, philosophical insight
- "Elemente der Psychophysik," preface, p. v.
- "German Psychology of To-day," Th. Ribot (where Delboeuf's researches are given). "The Human Mind," Sully, Vol. I.; article "Weber's Law" in the "Encyc. Britannica"; "Principles of Psychology," Wm. James, Vol. I. (a most unfavorable critique); "Elements of Physiological Psychology," Ladd; "Human and Animal Psychology," Wundt, Lectures II., III. and IV.; "Outlines of Psychology," Külpe.