i2 4 PSYCHO-PHYSICAL METHOD 



full effect in consciousness. Let us call this duration, 

 necessary for the production of the full effect, the 

 action-time of the light. This time is by no means 

 extremely brief; it is many hundred times longer than 

 the duration of a flash of lightning or of an electric 

 spark. That fact I can easily demonstrate to you 

 presently. What then is this duration in the case ot 

 a light of any given intensity ? If we can find the 

 answer to this question, other problems open up and 

 can be solved by modifications of the procedure. But 

 we will confine our attention to this first step of the 

 whole problem before us, the variation of the sensation 

 with the duration of the stimulus. 



It was first attacked by Helmholtz fifty years ago. 

 The plan he adopted was to take a disc of light, divided 

 into two equal parts of equal physical brightness, and 

 to expose both halves for a brief moment simultaneously 

 to the eye of the observer, the duration of the exposure 

 of the one half being slightly longer than that of the 

 other half. Then so long as the duration of exposure 

 of both halves is less than the action-time, the half which 

 is the longer exposed should appear the brighter, for 

 it has the longer time in which to produce its effects 

 in the retina, and will therefore more nearly produce 

 its full effect. If this is found to be the case, let the 

 duration of exposure of both halves be increased step 

 by step in successive experiments, until the difference 

 of brightness no longer appears. Then we may assume 

 that the duration reached when the difference of bright- 

 ness ceases to be perceptible is approximately the action- 

 time of that light. 



By this method a result was reached which, however, 

 was far from a correct measure of the action-time. To 

 what, then, was the error due ? You may be sure that 



