PSYCHO-PHYSICAL METHOD 123 



for it is realized now by an increasing number of thinkers 

 that the theory of organic evolution cannot be treated 

 as a branch of physical science ; that if, in considering 

 this great problem, we leave out of account the part 

 played by the mind, by its feelings, its desires, its power 

 of choice and its efforts, we are making an undue 

 simplification which will render impossible an approxima- 

 tion to a complete theory of evolution ; that, in fact, the 

 problem of animal evolution is distinctly a psycho-physical 

 problem a fact recognized by Darwin himself in his 

 doctrine of sexual selection and in his treatment of 

 instinct. 



Lastly, just because psycho-physics is the application 

 of the scientific method to the problem of the relation 

 of the physical and psychical worlds, a problem which 

 necessarily is dealt with by almost every speculative 

 or metaphysical writer, the psycho-physicist has to make 

 himself familiar with all such speculations, he has to 

 keep them in mind, and from time to time has to ponder 

 the bearing upon them of the positive results achieved 

 by his science. 



We turn now to illustrate the application of psycho- 

 physical method to a particular problem. 



The characters of a sensation excited by a stimulus 

 depend not only upon the quality and the intensity of 

 the stimulus, but also upon its duration, and the problem 

 I have chosen for illustration is this How does the 

 intensity of a visual sensation depend upon the duration 

 of the stimulus ? How does it vary with it ? 



If a ray of light of given intensity is allowed to act 

 on the retina for a brief moment only, the sensation it 

 excites is much less intense than if it is allowed to act 

 for a longer period ; i.e. the light must act iipon the retina 

 a certain length of time in order that it shall excite its 



