26 ROBERT M. YERKES 



The general apparatus need not be redescribed in detail. The 

 reader who is unfamiliar with it is referred to the above-men- 

 tioned monograph and to Watson's more recent book. 3 In 

 brief, it consists of a source of light which, by means of a system 

 of lenses, prisms, and slits, is made to supply chromatic stimuli 

 in any desired quality or intensity. Two stimuli are presented 

 to the subject simultaneously. The position of these stimuli 

 may be reversed at the will of the experimenter. The subject 

 is required to distinguish the stimuli and react differently to 

 the two. 



Assuming, now, that the reader has a general knowledge of 

 the mechanism by which the chromatic stimuli are obtained, 

 controlled, and measured, we may consider our method of pro- 

 cedure in its relations to the reaction-box of figure 1. This 

 consists of an entrance chamber (A) in which, at the beginning 

 of a series of observations, the subject is placed by the experi- 

 menter, and from which it passes, when the door (D) is raised, 

 into compartment B, which may be designated the discrimina- 

 tion compartment. A sliding partition (M) enables the experi- 

 menter to avoid delay because of the unwillingness of the subject 

 to enter B, for by raising the door (D) and drawing M slowly 

 and steadily backward toward the rear of compartment A, the 

 subject may, without disturbance, be compelled to enter the 

 discrimination compartment. Once in B, the subject faces the 

 two stimuli S, S. These are presented either with or without 

 general overhead illumination, and they appear as illuminated 

 surfaces, either chromatic or achromatic, 7 cm. long by 1.8 

 cm. wide. These two stimuli are separated by the partition 

 P, of figure 1. 



On the floor of each stimulus-box, E, are electrodes by means 

 of which electric shocks may be given as punishment for failures 

 to distinguish and properly to react to the two stimuli. The 

 doors F, F, leading from the stimulus compartments into the 

 alleys G, G, may be raised by the experimenter by means of 

 the cords shown in the figure. When the subject enters the 

 compartment which contains the stimulus selected by the ex- 

 perimenter as the positive stimulus, the appropriate door F is 

 immediately raised, the slide-door, H, of the same side opened, 

 and the subject thus permitted to pass by way of the alley G, 



3 Watson, J. B. Behavior. New York, 1914, p. 70. 



