PROGRESS IN EXPERIMENTAL PSYCHOLOGY 



ference between the stimuli. Thus if one holds an ounce 

 weight in each hand, and has tiny weights added to one 

 of them, grain by grain, one does not at first perceive a 

 difference ; but presently, on the addition of a certain 

 grain, he does become aware of the difference. Noting 

 now how many grains have been added to produce 

 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 repre- 

 sents 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 perceivable difference" re- 

 mained 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 difference" 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 



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