RESPONSE BY SENSATION 729 



well-known cases of nerve- disease, bringing on diminished 

 conductivity, this dissociation of sensation is met with patho- 

 logically. In paralysis, again, burning coals may be held in 

 the hand and induce only the feeling of contact, without any 

 sensation of pain. 



One very difficult problem in connection with psycho- 

 logical response is that of the peculiar relation between the 

 intensity of stimulus and that of response. The generalisa- 

 tion known as Weber-Fechner's law, asserts that stimulus 

 must increase in geometrical, for sensation to increase in 

 arithmetical, progression. Fechner, moreover, regarded this 

 relation, not as due to any physical or physiological factor, 

 but as a particular case of some specific psychological law. 

 On an inspection of the mechanical responses of animal 

 nerve, given in figs. 400, 401, and 402, however, we see that 

 the peculiar relation between stimulus and sensation follows 

 inevitably on the physiological character of those responses. 

 We there see that under feeble stimulus the response is 

 positive, connoting, as we know, a positive tone of sensation. 

 After this, as stimulus increases, the sign of response under- 

 goes a reversal into normal negative. From this point 

 onwards, for some time, the response to increasing stimulus 

 shows a rapid rate of increase ; but this increase tends to 

 reach a limit as the maximum molecular distortion is 

 approached. These facts follow naturally from the mole- 

 cular theory of response which has been described, and in 

 such considerations we obtain an explanation of those 

 changes in the tone or quality of sensation of which Weber- 

 Fechner's law was unable to take account. That these 

 responsive characteristics, again, are not peculiar to the 

 animal nerve, has been seen in the fact that vegetal nerves 

 also show a similar relation between stimulus and response 

 (fig. 403). That this relation indeed is universal, will be 

 understood from the response of an inorganic substance to 

 increasing stimuli, as given in fig. 404. 



Another interesting proof of the dependence of the 

 psychological upon physico-physiological changes is afforded 



