540 PHYSIOLOGY 



express in absolute measure the strength of a sensation. We cannot 

 even compare the strengths of two sensations differing in quality or 

 modality ; and although we can say that such and such a light is 

 stronger than another light, it is impossible to say that the sensation 

 resulting from the stronger is two, three, or more times that of the 

 weaker. In measuring the effect on sensation of increasing the stimulus 

 we are therefore reduced to using the smallest appreciable increase of 

 sensation as our unit of sensation. The question as to the relation 

 between the intensity of stimulus and the intensity of sensation resolves 

 itself into an inquiry as to what increase in a given stimulus is necessary 

 in order that it may evoke an appreciable increase in sensation. 

 Weber's law states that the increase of stimulus which is necessary to 

 produce an appreciable increase in sensation must always bear the 

 same ratio to the whole stimulus. Thus if we found that we could 

 just distinguish the difference between a weight of 10 oz. and a weight 

 of 9 oz., it would not be sufficient to add one ounce to a weight of 

 10 Ib. in order to produce a distinct difference in sensation. In the 

 latter case we should not be able to appreciate any difference until 

 we had added a pound, i.e. one-tenth of the whole stimulus to the 

 weight. We can distinguish between 10 oz. and 11 oz.. or between 

 10 Ib. and 11 Ib., but not between 10 Ib. and 10 Ib. 1 oz. 



Several methods have been proposed for testing the limits of the 

 applicability of this law. Of these the most important are : 



(1) The method of minimal difference. 



(2) The method of average error. 



In the first method we find by trial how much a given stimulus 

 must be increased in order to evoke an appreciaole increase of 

 sensation, and this determination is made for a number of stimuli of 

 different intensity. In the second method it is sought to find a 

 strength of stimulus which is just equal to another stimulus of given 

 intensity. It will be found that errors will be made on both sides, and 

 the average error is taken as representing the minimum difference, 

 which is just sufficient to cause a distinct difference of sensation. 



In all sense-organs Weber's law is only applicable between limits, 

 which vary with each sense-organ, and does not hold either for very 

 weak or for very strong stimuli. Within these limits the ratio which 

 an increase of stimulus must bear to the whole stimulus to produce an 

 increase of sensation may be given approximately as follows for the 

 different sense-organs : 



When weights are placed on corresponding points of two sides of 

 the body, e.g. on the two hands, we can appreciate differences of about 

 one-third ; if the contrast be successive, i.e. if the weights be placed on 

 the same spot in succession, we can appreciate differences between 

 one-fourteenth and one- thirtieth. The range over which this amount 



