484 



PHYSIOLOGY 



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 applica- 

 bility 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 appreciable 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 applicable only between limits which 

 vary with each sense organ, and it 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 in order 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 of accuracy is attained 

 extends from 50 to 1000 grammes. In judging of weights with the help of 



LATZNT PERIOD 

 OR THRESHOLD 



FIG. 242. Diagram to show relationship between stimulus and sensation. 



movement (the method one ordinarily adopts) the limit of accuracy is about 

 one- twentieth ; for sounds the appreciation of difference amounts to about 

 one-ninth, The organ which if most susceptible to slight changes of intensity 



