280 ADA W. YERKES 



could reenter the food box. The floor of the discrimination 

 boxes was covered with a series of copper wires connected with 

 a No. 6 Columbia dry cell and a Porter inductorium set at 5.75 

 on the scale. Used in daylight the experiment box was placed 

 facing a window so that when uncovered the two boxes received 

 the same amount of light. 



The first degree of difference in amount of light was made 

 when a piece of black cardboard, black side down, was placed 

 on top of one box and the other was left open. These were 

 designated, then, the dark, D, and the light, L, boxes. The 

 rats were required to choose the light box; if they chose the 

 dark one, they received a slight electric shock from the wires 

 on the floor of the box and were not allowed to pass through. 

 The dark side was changed from left to right irregularly but 

 according to a definite schedule printed on blanks on which the 

 records were kept. The time from entrance through the swing- 

 ing wire door into the chamber facing the dark and light boxes 

 until the choice was made was taken by means of a stop watch. 

 It was called a choice when an animal placed both forefeet far 

 enough across the wires to receive the shock, if the choice were 

 wrong. This discrimination time varied from four-tenths seconds 

 to more than thirty minutes when the animals were timid or 

 discrimination was difficult. The total time required for the 

 regular daily series of five or ten trials was also noted. 



In order to teach the method of procedure and also to test 

 possible preference for dark or light, two preliminary series, 

 A and B, of ten trials each were given without shock and with 

 the dark box open for free passage through. Each regular series 

 thereafter consisted of five trials, and one series was given a 

 day, at the same hour of the morning. One rat was in the 

 apparatus at a time. They were not hurried but were gently 

 and firmly directed through the entrance doors, allowed to 

 choose, guided back, if necessary, to the food box, and allowed 

 to eat for 15-30" before the next trial. The readiness with 

 which the rats learned of their own initiative to pass through 

 the swinging wire doors was used as a measure of their degree 

 of initiative. This is discussed in the next section. Bread 

 crumbs soaked in milk and mixed grains were the food used 

 in the food box. After the series for the day was completed the 

 animals were allowed to feed for half an hour in another cage 

 before being returned to their own cage. 



