PSYCHO-PHYSICAL METHOD 121 



along the optic nerve to the brain ; and it is not until 

 this change, this nervous-impulse as it is called, reaches 

 a particular spot on the surface of the brain that the 

 sensation springs into existence. If the nervous elements 

 in this part of the surface of the brain are destroyed, 

 or the nerve fibres connecting them with the sense- 

 organ are interrupted in any way, the physical stimulus 

 will fail to excite the sensation. 



The relation between the sensation and the physical 

 stimulus that excites it is therefore, although fairly 

 constant and definite, very indirect; it depends upon 

 the integrity of groups of nervous elements in the 

 sense-organ and in the brain, and of a chain of elements 

 connecting them together; and the immediate psycho- 

 physical relation is that between the sensation and certain 

 nervous processes in the substance of the brain. 



Before, then, we can obtain any deep insight into the 

 nature of this relation, or can reach any such conclusion 

 as Fechner sought, we must know far more than we at 

 present do of these delicate and complex processes that 

 go on in the brain and in the nerves and in the sense- 

 organs. Hence psycho-physics allies itself intimately with 

 the study of these processes, the physiology of the nervous 

 system and sense-organs, and thankfully accepts all the 

 facts that the physiologists have been discovering at 

 a constantly increasing rate ever since Johannes Miiller 

 in Germany and Sir Charles Bell in this country set 

 this branch of research on the path of rapid progress 

 nearly a century ago. And it is not content to receive 

 only from physiology the application of its methods 

 has thrown new light on many purely physiological 

 problems. 



With empirical introspective psychology, for so long 

 pursued as an isolated branch of science, psycho-physics 



