16 ROBERT M. YERKES 



As originally constructed, no provision was made in the apparatus 

 for locking the entrance and exit doors of the several boxes when 

 they were closed. But as two of the subjects after a time learned 

 to open the doors from either outside or inside the boxes, it 

 became necessary to introduce locking devices which could be 

 operated by the experimenter from the observation bench. This 

 was readily accomplished by cutting holes in the floor, which 

 permitted an iron staple, screwed to the lower edge of each door, 

 to project through the floor. Through these staples by means 

 of a lever for each of the nine boxes, the observer was able to 

 slide a wooden bar, placed beneath the floor of the room, thus 

 locking or unlocking either the entrance door, the exit door, or 

 both, in the case of any one of the nine boxes. 



Since figure 17 is drawn to scale, it will be needless to give 

 more than a few of the dimensions of the apparatus. Each of 

 the boxes was 42 inches long, 18 inches wide, and 72 inches deep, 

 inside measurements. The alleys D, I, and H were 24 inches, 

 and G 30 inches wide, by 6 feet deep. The doors of the several 

 boxes were 18 inches wide, by 5 feet high, while those in the 

 alleyways were 24 inches wide by 6 feet high. The response- 

 compartment E of figure 17 was 14 feet 4 inches, by 8 feet, by 

 6 feet in depth. In order that the apparatus might be used with 

 adult human subjects conveniently, if such use should prove 

 desirable, the depth throughout was made 6 feet, and it was 

 therefore possible for the experimenter to walk about erect in it. 



The experimental procedure was briefly as follows: A small 

 quantity of food having been placed in each of the food cups 

 and covered by the metal flanges on the exit doors, the experi- 

 menter raised door 11 of figure 17 and then opened door 10 and 

 the door of the cage in which the desired subject was confined. 

 After the latter, in search of food, had entered the runway D, 

 the experimenter lowered door 11 to keep it in this runway, and 

 immediately proceeded to set the reaction-mechanisms for an 

 experiment (trial). Let us suppose that the first setting to be 

 tried involved all of the nine boxes. Each of the entrance doors 

 would therefore be raised. Let us further suppose that the right 

 door is defined as the middle one of the group. With the appa- 

 ratus properly set, the experimenter next raises door 12, thus 

 admitting the animal to the response-compartment E. Any 

 one of the nine boxes may now be entered by it. But if any 



