254 



THE SPECIAL SENSES. 



measurement. Weber, in studying the relation between the 

 strength of the stimulus and the amount of the resulting sensation, 

 availed himself of the method of the least detectible change in sen- 

 sation ; that is, he determined the increase in stimulus at different 

 levels necessary to cause a just perceptible increase in the sensation. 

 By means of this method he arrived at the significant result that 

 the increase in stimulus necessary to cause this change is, within 

 physiological limits, a definite fractional increment of the acting 

 stimulus. If, for instance, with a weight of 30 gms. upon 

 the finger it requires an increment of -g^ that is, one additional 

 gram to make a just perceptible difference in the pressure sensa- 



Fig. 111. Curve to indicate the Weber-Fechner law of a logarithmical relation be- 

 tween excitation and sensation. (From Waller.) The excitations are indicated along the 

 abscissas, the sensations along the ordinates. The increase in sensation is represented as tak- 

 ing place in equal steps, "the minimal perceptible difference," while the corresponding 

 excitations require an increasing increment of $ at each step, namely 1, 1.33, 1.77, 2.37, 

 etc. That is, for equal increments of sensation increasing increments of stimulation are 

 necessary. 



tion; then, with a weight of 60 gms. upon the finger the addition 

 of another gram would not be perceived ; it would require again an 

 increment of -^ that is, 2 gms. to make a just perceptible dif- 

 ference in sensation. This relationship is known as Weber's law. 

 While its exactness has often been disputed, it seems to be generally 

 admitted that for a median range of stimulation the law expresses 

 the approximate relation between the two variables considered. 

 Fechner attempted to give this law a more quantitative and ex- 

 tensive application by assuming that just perceptible differences 

 in sensation represent actually equal amounts of sensation. Ac- 

 cepting this assumption, we can express the relationship between 

 stimulus and sensation as determined by Weber's experiments by 



