DISCRIMINATIVE RESPONSES TO VISUAL STIMULI 



467 



or L 2 as the case may be. The subject holds in either hand 

 a rubber bulb, which when quickly pressed, actuates a Dunlap 

 pneumatic reaction key, to which it is attached by a rubber 

 tube. 8 The reaction consists in pressing the bulb corresponding 

 to the side of the field which is darkened, as soon as the darkening 

 is perceived. A warning signal, made by an electrical buzzer, 

 is given by hand from 1 to 1.5 seconds before the stimulus is 



M, 



"% L 



FIG. 1 



r, integrating polyhedron; E, position of eye of observer; W, limiting window 

 P, 10 degree double prism; MG$, milk glass surface forming standard portion of 

 photometric field; L S) tungsten lamp illuminating MG 3 ; Mi and M 2 , silvered 

 mirrors; MG\ and MG 2 , milk-glass surfaces viewed from E by reflection 

 from Mi and M* and by partial reflection from P; L t and L 2 auxiliary tungsten 

 lamps, illuminating MGi and MG 2 respectively, the extinction of which is the 

 stimulus to reaction. Sundry protective screens used to arrest scattered light, 

 are not shown. 



presented, the time depending on the desire of the subject, and 

 being kept as nearly constant as possible. 



In ten to fifteen cases, irregularly distributed within each 

 series of 100 reactions, the warning signal is not followed by the 

 darkening of either side of the field, and the subject is instructed 



8 The keys used in this work were modified so as to constitute double-throw 

 switches. Thus, if the wrong key was actuated, it momentarily closed a signal 

 circuit (shown in figure 2). The range of movement of the common connector 

 in the reaction switch was made less than 1 mm., in order to insure closing of the 

 signal circuit. 



