120 PSYCHO-PHYSICAL METHOD 



difference between pairs of sensations. (4) We can note 

 the quantity of difference between two stimuli that just 

 suffices to produce a perceptible difference between the 

 corresponding sensations. 



All such judgements of likeness and of difference are, 

 however, liable to be affected and rendered variable by 

 very many factors, both physiological and psychological, 

 and Fechner therefore, starting out with these four 

 possibilities of measurement, proceeded to devise and 

 elaborate methods by which this variability and uncer- 

 tainty might be eliminated. The methods which he 

 devised are known as the psycho-physical methods ; they 

 are indispensable for almost all exact psycho-physical 

 experiment, and they have been applied with success in a 

 great number of instances. 



I propose to try to make clear to you presently the 

 nature of one of these methods by illustrating its appli- 

 cation to a particular problem, with the aid of a simple 

 apparatus. But before doing that, I must say a little 

 about the relation of psycho-physics to its allied sciences. 



Fechner's great enterprise failed for this reason chiefly. 

 He was attempting to establish quantitative relations 

 between psychical magnitudes and physical magnitudes 

 with which they are related, between intensities of sensa- 

 tion and the magnitudes of the physical stimuli that 

 excite them. But we know now that this relation is 

 not a direct one. Between the physical stimulus, say 

 the ray of light of given intensity falling on the eye, 

 and the resulting sensation of colour there intervenes 

 a long chain of processes that take place in the sense- 

 organ and the nervous system. The ray of light 

 induces some chemical or physico-chemical change in 

 the retina ; this in turn induces some chemical or physical 

 change in the optic nerve, which change is propagated 



