EXPERIMENTAL PSYCHOLOGY 



certain grain, he does become aware of the difference. 

 Noting now how many grains have been added to pro- 

 duce this effect, we have the weight which represents 

 the least appreciable difference when the standard is 

 one ounce. 



Now repeat the experiment, but let the weights be 

 each of five pounds. Clearly in this case we shall be 

 obliged to add not grains, but drachms, before a differ- 

 ence between the two heavy weights is perceived. 

 But whatever the exact amount added, that amount 

 represents the stimulus producing a just-perceivable 

 sensation of difference when the standard is five pounds. 

 And so on for indefinite series of weights of varying 

 magnitudes. Now came Weber's curious discovery. 

 Not only did he find that in repeated experiments with 

 the same pair of weights the measure of "just- er- 

 ceivable difference" remained approximately fixed, 

 but he found, further, that a remarkable fixed relation 

 exists between the stimuli of different magnitude. If, 

 for example, he had found it necessary, in the case of 

 the ounce weights, to add one-fiftieth of an ounce to the 

 one before a difference was detected, he found also, in 

 the case of the five-pound weights, that one-fiftieth of 

 five pounds must be added before producing the same 

 result. And so of all other weights ; the amount added 

 to produce the stimulus of "least-appreciable differ- 

 ence" always bore the same mathematical relation to 

 the magnitude of the weight used, be that magnitude 

 great or small. 



Weber found that the same thing holds good for the 

 stimuli of the sensations of sight and of hearing, the 

 differential stimulus bearing always a fixed ratio to the 



265 



