THE STORY OF NINETEENTH-CENTURY SCIENCE 



which hitherto had failed to attract the attention it de- 

 served. The method consisted of the measurement and 

 analysis of the definite relation existing between exter- 

 nal stimuli of varying degrees of intensity (various sounds, 

 for example) and the mental states they induce. Weber's 

 experiments 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 pain- 

 fully audible in the silence of the night. Yet again, the 

 difference between an ounce weight and a two- ounce 

 weight is clearly enough appreciable when we lift the 

 two, but one cannot discriminate in the same way be- 

 tween a five -pound weight and a w eight of one ounce 

 over five pounds. 



This last example, and similar ones for the other senses, 

 gave Weber the clew to his novel experiments. Reflec- 

 tion 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 com- 

 parison 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- 



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