PSYCHO-PHYSICAL METHOD 131 



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Hence you find that in many lighthouses, especially 

 those on the coast of France, where the engineers have 

 taken up this improvement eagerly, the flashes of the 

 revolving lights are very brief. But in order that the 

 maximum economy and efficiency of the light may 

 be achieved, the engineer must know just what we 

 have called the action-time of the light, the duration 

 of the flash necessary for the production of its full 

 effect on the eye and the consciousness of the ob- 

 server. This knowledge has only been put at the dis- 

 posal of the engineers by the development of psycho- 

 physical method ; and this development we owe entirely 

 to the spirit of pure science which, embodied in its 

 loftiest form in Fechner and his successors, set out to 

 apply the principles of scientific method to the investiga- 

 tion of the relation of the physical to the psychical 

 world : that great problem which for so long had 

 remained as a subject of speculation only, but which is 

 now being brought within the pale of science by methods 

 of which I have tried to give you a glimpse in this 

 lecture. 



K 2 



