PHYSICAL BASIS OF SENSATION 655 



tive (pleasure-pain). In this region, then, it is theoretically 

 possible to bring out the positive element alone, by suppress- 

 ing the negative. It is the predominance of either one of 

 the two components which at any given moment determines 

 the pleasure-pain character of the complex sensation. At 

 a certain critical point it i's the element of pain which will 

 begin to appear as relatively conspicuous, this proceeding 

 towards a climax with increasing stimulation. 



It must be understood that we are dealing in general 

 with nervous response under normal conditions of excitability. 

 It will be sufficient here to make a cursory reference to 

 certain exceptional cases which may occur. We have found 

 that the character of the response given by a tissue is deter- 

 mined by the two factors of (i) the effective intensity of 

 stimulus, and (2) the excitability of the responding tissue. 

 If the nature of a given stimulus be such as to produce but 

 a moderate effect, there will be a greater likelihood of its 

 evoking only positive response. Or the responding tissue, 

 in another case, may possess exceptional excitability ; hence 

 the responsive indication here will tend from the beginning 

 to be negative. The different effects depending on the vary- 

 ing excitatory characteristics of the tissue, we have already 

 seen illustrated in several cases. The slightly excitable 

 epidermis was seen to give a predominantly positive response, 

 whereas nerve which had been rendered highly excitable 

 tended, on the other hand, to give negative response. 



We shall now proceed further to consider the corre- 

 spondence between the responsive sensation and the degree 

 of nervous change induced by stimuiws. And here the first 

 question that arises is that of the relation which sensation 

 bears to the intensity of the stimulus that provokes it. The 

 difficulty of this investigation lies in the generally unsatis- 

 factory character of the subjective standard, and in the 

 variations induced by stimulus itself in the sensitiveness of 

 the neurile elements. 



According to what is known as Weber-Fechner's Law, 

 the strength of stimulus must increase in geometrical ratio 



