518 PHYSIOLOGY 



of application of the stimulus and the moment at which the reaction 

 takes place. Thus, if the reaction time for an auditory stimulus is to 

 be determined, the electric current is arranged so as to pass through : 



(1) A spring contact key which can be pressed so as to make a 

 noise. 



(2) An electric signal writing on a rapidly moving surface. 



(3) A second key, which the subject will release as soon as he 

 hears the noise of the first key and so break the current. 



The recording surface may be a drum, a pendulum myograph, 

 or a spring myograph, such as the ' shooter ' of du Bois-Raymond. 

 If the sensory impression is to be from the skin the current may 

 be made to pass through the primary coil of an inductorium, and 

 wires be taken from the second coil to some part of the surface of 

 the skin. In this case the signal may be started by opening the 

 circuit, and the subject of the experiment will respond by closing the 

 circuit by means of a spring key directly he feels the shock caused by 

 the break of the primary circuit. If the reaction period is to be 

 determined for sight a white piece of paper may be placed on an 

 electro-magnet in the primary circuit and the person will respond 

 directly he sees this move. Many other instruments have been devised 

 for the same purpose. The average reaction times obtained with the 

 different senses are as follows : 



Electrical 

 Sight Hearing stimulation of skin 



0-186 to 0-222 sec. 0-115 to 0-182 sec. 0-117 to 0-201 sec. 



The two figures given for each case are the extremes obtained 

 in different series of observations. 



The times vary according to the condition of the person that is the 

 subject of the experiment. They are lengthened by fatigue ; they 

 are shortened up to a certain point by continued practice. Within 

 limits also they are shortened by increase of the strength of the stimulus. 



DILEMMA. When the subject has to make a deliberate choice 

 between the parts of the body stimulated the reaction time is con- 

 siderably longer. To show this, the wires from the secondary coil are 

 connected by a switch to two pairs of electrodes which are applied, 

 one to the right and one to the left half of the body. It is agreed 

 beforehand that the subject shall react only to stimulation, say, 

 of the right side. The switch is removed from the observation of 

 the subject and the stimulus is applied irregularly to one side or to 

 the other. It is found that the additional neural processes involved 

 in determining whether the stimulus is on the right side, and therefore 

 should be followed up as agreed, adds considerably to the length 

 of the reaction time (on an average '066 sec.). It is possible to 



