256 HUMAN PHYSIOLOGY 



It is to be noticed that the adequate stimulus does not objec- 

 tively contain the quality of the sensation which it produces. For 

 example, the vibrations of ether which act upon the eye have 

 nothing to do with the notion of light. The conception of light 

 consists only in subjective perception. 



The intensity of the sensation is, other things being equal, 

 dependent upon the intensity of the stimulus. 



The liminal intensity of a stimulus is the feeblest stimu- 

 lus still perceptible; the " difference-threshold " of the 

 stimulus is the smallest perceptible difference in the intensity 

 of two stimuli or the smallest perceptible change in a stimu- 

 lus. The size of the ' ' difference-threshold ' ' varies with the 

 absolute strength of the intensity of the stimulus. The 

 smallest perceptible change in the intensity of the stinuilns is 

 proportional to the absolute strength of the stimulus \Veber* s 

 Law. 



According to Fechner's psycho-physical law, the strength of 

 the sensations is related to the strength of the stimuli as a 

 logarithm to its number. The validity of Fechner's law is dis- 

 puted by many authors. 



1 Objections have also been made against the general validity of 

 Weber's la\v. 



Besides differences in intensity and quality we can discrimi- 

 nate between the duration of sensations, and in some (sight 

 and tactile) between the space conditions (place and extent 

 of sensation). 



