CH. LII.] WEBER'S LAW 765 



ing sensory centres are quiescent. The balance of evidence, however, 

 is very strongly against this view. It is better to suppose that the 

 physiological processes underlying a sensation and its revived 

 memory image are broadly the same. There is unquestionably 

 some physiological difference corresponding to the difference between 

 sensory experiences and hallucinations on the one hand, and revived 

 experiences on the other. But at present it is impossible to say in 

 what that difference consists. 



When, as occurs under certain conditions, an object is adjudged 

 different from what general experience teaches us to be its " real " 

 character, we have an illusion. Thus a line or figure may appear to 

 be longer or shorter than it really is, or to take a direction different 

 from its real direction. Or a weight may appear heavier than 

 another which is really equal to it. Illusions are due partly to 

 peripheral, partly to central factors. Their investigation falls within 

 the province of experimental psychology. 



