PSYCHO-PHYSICAL METHOD 125 



the error was not due to any defect in the construction 

 or working of the apparatus. As we should expect, the 

 physical conditions of the experiment were faultless. 



But it is a peculiarity of psycho-physical experiment 

 that in every case you have to satisfy conditions of three 

 kinds, namely, physical, physiological and psychological 

 conditions. Perfection of the physical conditions avails 

 nothing if these other conditions are faulty. And by 

 Helmholtz's procedure the physiological and the psycho- 

 logical conditions were not satisfied. 



The physiological conditions were unfavourable be- 

 cause, as we now know, when two parts of the retina 

 are simultaneously but unequally excited, there takes 

 place an interference between the two nervous processes, 

 and this results in modification of the intensities of the 

 resulting sensations, the one being rendered more, the 

 other less, intense, an effect which we call a contrast- 

 effect. 



The psychological conditions were unfavourable be- 

 cause, in order to effect an accurate comparison of the 

 intensities of two sensations, it is necessary to turn the 

 attention first to one then to the other, and this was 

 impossible when the two momentary sensations were 

 simultaneously excited. 



The imperfection of the procedure illustrates a truth 

 of great importance. It may seem to you strange that 

 the genius of Helmholtz and his great experience and 

 ingenuity in all kinds of optical experiment should not 

 have enabled him to devise at once a perfect method. 

 But every man, no matter how great his genius, is more 

 or less dependent upon the state of science and of 

 scientific method that has been worked out by his 

 predecessors. And when Helmholtz devised these 

 experiments the psycho-physical methods of experiment 



