A HISTORY OF SCIENCE 



grew out of the familiar observation that the nicety 

 of our discriminations of various sounds, weights, or 

 visual images depends upon the magnitude of each 

 particular cause of a sensation in its relation with other 

 similar causes. Thus, for example, we cannot see the 

 stars in the daytime, though they shine as brightly then 

 as at night. Again, we seldom notice the ticking of a 

 clock in the daytime, though it may become almost 

 painfully audible in the silence of the night. Yet 

 again, the difference between an ounce weight and a 

 twQ-ounce weight is clearly enough appreciable when 

 we lift the two, but one cannot discriminate in the same 

 way between a five-pound weight and a weight of one 

 ounce over five pounds. 



This last example, and similar ones for the other 

 senses, gave Weber the clew to his novel experiments. 

 Reflection upon every-day experiences made it clear 

 to him that whenever we consider two visual sensations, 

 or two auditory sensations, or two sensations of weight, 

 in comparison one with another, there is always a limit 

 to the keenness of our discrimination, and that this 

 degree of keenness varies, as in the case of the weights 

 just cited, with the magnitude of the exciting cause. 



Weber determined to see whether these common ex- 

 periences could be brought within the pale of a general 

 law. His method consisted of making long series of ex- 

 periments aimed at the determination, in each case, of 

 what came to be spoken of as the least observable dif- 

 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 per- 

 ceive a difference; but presently, on the addition of a 



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