VISION IN THE MOUSE 



559 



Method: The method is very similar to that previously des- 

 cribed. Two animals were used : 



These animals were trained to enter the brighter of the two 

 compartments. Food was kept in both alleys near 0, but the 

 exit into the alley on the side of the weaker light was closed by a 

 piece of glass which could be slipped into the partition just above 

 the exit and pushed down. A shock was given whenever the 

 mouse entered the darker compartment. From the brighter com- 

 partment the animal was allowed to pass unmolested into the alley, 

 obtain food and enter the nest box through the passage 0. After 

 receiving a shock the animal was apt to run quickly out of the 

 darker compartment and into the other one. 



The wire door at / permitted passage only into B, and those 

 at 0, 0, only into .4. The mice soon learned that the food was to 

 be obtained only by passing from B through one of the small 

 compartments into the alley. 



Twenty experiments were made with a mouse each day. Some- 

 times it was found necessary to urge the animal to make a choice 

 by gradually narrowing the space in compartment B with a thin 

 board, placed vertically across the compartment. 



The experiment was begun with one light at a distance of 34 cm. 

 from the ground glass, and the other at 54 cm. After each choice, 

 or sometimes two or three choices, the light that had been at 34 

 cm. was moved to 54 cm. and the other moved up to 34 cm. thus 

 making the other compartment the brighter. At the same time 

 the piece of glass which had blocked the exit was removed and 

 pushed into the other slip, blocking the exit on the opposite side. 



Results : ("Right" indicates choice of the brighter of the two 

 lights). 



