THE SENSE ORGANS 483 



with increasing strength of stimulus. In each sense organ we can measure 

 the amount of energy which must be applied to it in order to evoke a minimum 

 sensation. This figure varies considerably with the physiological condition 

 of the animal. In dealing with reflexes we have seen that the motor result 

 of stimulation of a receptor organ varies in the same manner. Thus a mini- 

 mal stimulus is more effective if repeated a few times at definite intervals 

 (summation of stimulus) : the stimulus which is subminimal may become 

 minimal and effective as a result of repetition. 



Another factor which intervenes is that known as ' adaptation ' a 

 process associated to a certain extent with the phenomenon of fatigue. 

 Adaptation is best studied in the case of the eye. Here the dark-adapted 

 eye, i.e. one that has been kept from light for half an hour, will react, and 

 give a visual sensation, to a strength of stimulus which is only one-fiftieth 

 of the minimal stimulus required to evoke sensation in the eye that has been 

 lately exposed to light. 



i Another phenomenon which may alter the strength of the liminal intensity 

 of stimulus is that known as ' contrast.' A finger plunged into mercury 

 feels a ring of constriction at the level of the surface of the mercury, i.e. 

 where there is a contrast between the pressure of the mercury and the 

 absence of pressure as the finger emerges into the air. 



The strength of the effective stimulus depends also on the number of 

 nerve-endings simultaneously excited. Thus when dealing with tactile 

 sensations, or sensations of pressure, in determining the minimal stimulus, 

 we must take into account the area stimulated, and we express the stimulus 

 just sufficient to produce a threshold sensation, as ' weight per square milli- 

 metre of surface.' Moreover the rapid ' fatigability ' or adaptation of all 

 sense organs makes the rate at which the stimulus is applied of considerable 

 importance. 



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 abso- 

 lute 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 sensa- 

 tion 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 sensa- 

 tion 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 differ- 

 ence between a weight of 10 oz. and a weight of 9 oz., it would not be sufficient 



