456 ON SENSATIONS IN GENERAL 



Between the minimum and the maximum, as thus defined, variations in 

 the strength of the stimulus will produce variations in the intensity of the 

 sensation. 



1. WEBER'S LAW 



In estimating differences in the intensity of sensations we meet with a 

 peculiar difficulty. We can say that a certain sensation is stronger or weaker 

 than another of the same kind, but we cannot say how much stronger or 

 weaker it is; for every sensation constitutes a whole in itself and cannot 

 be represented as the sum of several individual sensations. If, for example, 

 a white surface is illuminated at one time by 1 candle power and at another 

 by n candle powers, we can say perhaps that the sensation in the second case is 

 stronger than in the first; but we cannot tell how much stronger it is. 



The relation between the strength of the stimulus and the intensity of the 

 sensation is not to be determined by merely setting arbitrary stimuli over 

 against the sensations evoked by them. We can better approach the problem 

 by inquiring how much a given stimulus must be changed to produce a per- 

 ceptible change in the intensity of the sensation. E. H. Weber who made the 

 first observations along this line (1831) laid down the following law known 

 by his name : The increase in the stimulus necessary to produce a perceptible 

 change in a given sensation must always bear the same relation to the size 

 of the initial stimulus. 



Thus, if to a weight of 1 unit a person must add a weight of ^Vth i n order 

 to make the second load perceptibly heavier in his own subjective appreciation 

 of weight than the first, then, according to Weber's law, with an initial load 

 of 10 the superadded weight must be J^-ths to enable him to detect the dif- 

 ference. 



By placing weights on both hands at the same time, the hands being sup- 

 ported on the table, Weber found that the " threshold difference " was one-third 

 of the initial stimulus, but when the same hand was successively weighted it 

 was only one-fourteenth to one-thirtieth of the initial stimulus. In estimating 

 weights by the muscular sense i. e., by lifting them the threshold difference 

 goes down to one-fifteenth to one-twentieth when both hands are used simul- 

 taneously, and to one-fortieth when the weights are lifted successively with the 

 same hand. 



According to experiments by Merkel with fairly pure pressure stimuli, the 

 threshold difference for 50, 100, 200, 500 and 1,000 g. was y^, 7-^, T -^, 

 -gV , and i ^ 6 respectively. For weights above and below these values it is 

 not so constant. 



Another illustration of the law is the following : When one looks at a draw- 

 ing with shadings under different degrees of illumination, the fine gradations 

 of brightness come out with about the same clearness. For example, if he look 

 at the drawing first with the naked eye, then through a gray glass which 

 diminishes the intensity of the light rays from different parts of the drawing to 

 the same extent proportionally, the different parts of it are still seen in their 

 proper relations as regards light and shade. This would not be true if the 

 same proportional decrease in the intensity of the stimuli coming from dif- 



