DISCRIMINATION AND COMPARISON. 525 



to Wundt and which Wundt had rejected. This is the 

 einfache Wahlmethode, as Wundt calls it. The reacter 

 awaits the signal and reacts if it is of one sort, but omits to 

 act if it is of another sort. The reaction thus occurs after 

 discrimination ; the motor impulse cannot be sent to the 

 hand until the subject knows what the signal is. The 

 nervous impulse, as Mr. Cattell says, must probably travel 

 to the cortex and excite changes there, causing in conscious- 

 ness the perception of the signal. These changes occupy 

 the time of discrimination (or perception-time, as it is called 

 by Mr. C.) But then a nervous impulse must descend from 

 the cortex to the lower motor centre which stands primed 

 and ready to discharge ; and this, as Mr. C. says, gives a 

 will-time as well. The total reaction-time thus includes 

 both ' will-time ' and ' discrimination-time.' But as the 

 centrifugal and centripetal processes occupying these two 

 times respectively are probably about the same, and the 

 time used in the cortex is about equally divided between 

 the perception of the signal and the preparation of the 

 motor discharge, if we divide it equally between percep- 

 tion (discrimination) and volition, the error cannot be 

 great.* We can moreover change the nature of the per- 

 ception without altering the will-time, and thus investigate 

 with considerable thoroughness the length of the percep- 

 tion-time. 



Guided by these principles, Prof. Cattell found the time 

 required for distinguishing a white signal from no signal 

 to be, in two observers : 



0.030 sec. and 0.050 sec; 

 that for distinguishing one color from another was simi- 

 larly : 



0.100 and 0.110; 



that for distinguishing a certain color from ten other col- 

 ors : 



0.105 and 0.117 ; 



that for distinguishing the letter A in ordinary print from 

 the letter Z : 



0.142 and 0.137 ; 



* Miud, XI. 379. 



