128 PSYCHO-PHYSICAL METHOD 



2 and 10 in width respectively. The light of alternate 

 flashes is then of the same physical intensity but of 

 different duration, and you see that the flashes appear 

 alternately brighter and less bright. 



Now for the procedure to determine the action-time 

 of light of this particular intensity and quality. 



We make both windows so narrow that the duration 

 of each flash shall be less than the action-time, which we 

 first determine very roughly by trial and error. And we 

 make one window just so much narrower than the other 

 that the difference of brightness of the two flashes is 

 just distinctly perceptible. 



We then increase the width of both by small steps of 

 equal proportional extent, until we reach such a width 

 that the longer flash no longer appears the brighter. 



We know then that the duration of the longer flash 

 slightly exceeds the action-time ; for as soon as the dura- 

 tion of the longer flash equals the action-time, a further 

 increase of its duration no longer increases its bright- 

 ness, whereas a similar increase of duration of the briefer 

 flash still increases its brightness ; therefore, when this 

 stage has been reached, the just perceptible difference 

 of brightness between the two flashes is abolished by 

 a further small increment of duration of each flash. 



Here we must note an important feature of psycho- 

 physical method by means of which we can further 

 refine our determination of the action-time. 



We cannot arrive at a certain conclusion as to the 

 presence or absence of a just perceptible difference of 

 brightness by a single comparison. The internal factors 

 that affect the accuracy of our judgement are so many and 

 so variable, that a difference of stimulus which at one 

 moment suffices to determine a judgement of difference 

 may fail to do so at another moment ; e.g. the attention 



