484 PHYSIOLOGY 



sound was heard when a weight 6'7 milligrammes was allowed to fall a 

 distance of 1-2 mm. on to an iron plate at a distance of 500 mm. 



(6) VISION. The minimum intensity of light necessary to arouse sensa- 

 tion in a dark- adapted eye is, according to Aubert, equal to about one 

 three-hundredth of the intensity of the light reflected from a piece of white 

 paper which is being lit by the light of the full moon. The amount of 

 energy involved in such a stimulus is much smaller even than that deter- 

 mining a sensation of touch or hearing. 



WEBER'S LAW 



It is an interesting question how far the strength of sensation may be 

 regarded as an index to the strength of stimulus. Although it is easy to 

 measure in absolute terms the intensity of a stimulus, i.e. of a purely physical 

 process, there is no means by which we can 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 sensa- 

 tion. 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 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 only applicable between limits, which 



