PRACTICAL PHYSIOLOGY 75 



from the person experimented on. The latter sits 

 at the table with one finger resting lightly on one of 

 the levers. He is to respond by pressing the lever 

 the instant he (a) feels a tap on that finger, his eyes 

 being shut ; (fr) hears a tap on the second lever ; (c) 

 sees the movement which is imparted to the second 

 lever by the experimenter, who presses it down on 

 the other side of the screen. In each case two 

 marks are recorded upon the abscissa, one being 

 that which is made by the experimenter in impart- 

 ing the stimulus and the other that made by the 

 observed person in responding. The interval be- 

 tween the two marks, which can be accurately 

 measured by the aid of a tuning-fork tracing, indi- 

 cates the time between stimulus and response 

 i.e., the reaction time in the case of each of the 

 three senses. To record this with any accuracy a 

 large number of observations must be made with 

 each method of stimulation and the average time 

 taken. 



Discrimination time. For the measurement of 

 this the observed person places one finger upon 

 the end of each lever. It is agreed beforehand 

 that he is only to react to a stimulus received on 

 the one side, not on the other. The experimenter 

 may stimulate either. It will be found that the 

 reaction time is lengthened by a certain interval, 

 and this increase of reaction time is termed the 

 discrimination time. 



Volitional time. Similar observations are 

 made, but with the understanding that it is only 

 the hand on the side which receives the stimulus 



