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EXPERIMENTAL PHARMACOLOGY 



The operator holds key B. The drum is started off at a 

 fast rate. While the subject watches the signal magnet 

 closely the operator suddenly closes key B. The writing 

 point of the signal magnet at once goes down. Instantly 

 as the subject sees this he opens key A. The drum keeps 

 running, the operator opens key B and the subject again 

 closes key A. The operator now closes key B and the sub- 



K Key A 



Fig. 134. Arrangement of apparatus for recording reaction time for sight. 



ject instantly opens key A. In this manner about twenty 

 or thirty normal records are taken. With a vibrating 

 tuning fork (Fig. 136) beating fiftieths or hundredths of 

 a second, a time tracing (three or four may be needed) is 

 now run around the drum. With a pair of dividers or a 

 rule the average length of time which it takes the subject 



