78 POPULAR SCIENCE MONTHLY 



the generalization to which the name " Weber's Law," or the " Fechner- 

 Weber La.w," or the " Psycho-physical Law/' has been given. Re- 

 ferring to this discover}-, in the preface to the first great book on phys- 

 iological 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." 10 

 The law summarizes mathematically the relation between physiological 

 stimulus and psychical sense-perception. It is based on the fact, fami- 

 liar in common experience, and now authenticated by numerous observa- 

 tions 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 meas- 

 uring 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 per- 

 ceptible 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 interpreta- 

 tions, 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 litera- 

 ture. 11 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 



10 " Elemente der Psychophysik," preface, p. v. 



u " 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," Kiilpe. 



