REACTIONS OF CHICKS TO OPTICAL STIMULI 293 



describe this chick as rather undersized, strong, active, sensitive, 

 cautious, and quiet. Detailed records of its reactions show that 

 selection of stimuli was made by affirmation and negation most 

 frequently, by comparison occasionally. 



REACTIONS TO SIZES 



The method of testing size discrimination was similar to that 

 followed in the tests on forms. Two circular areas of different 

 size were presented to the chicks, one 5 cm. in diameter, the 

 other 8 cm. In the sliding frame of the apparatus an 8 cm. 

 circle was mounted at each end, a 5 cm. circle in the middle. 

 In other respects the appointments of the apparatus were the 

 same as in the form experiments. 



In these tests three factors were taken into consideration, any 

 one of which might have become a means of discrimination: 



1. Difference in size of the lighted areas, 



2. Difference in brightness of the lighted areas, and 



3. Difference in brightness of the right and left sides of the 

 experiment box. 



To eliminate other means of discrimination but the differ- 

 ence in size of the lighted surfaces, the following list of tests 

 was adhered to throughout the training: 



1 . Sides of box equal in brightness : 



2 . Sides of box equal in brightness : 



3 . Circles equal in brightness : 



4. Circles equal in brightness: 



5. Large circle the brighter: 



6. Small circle the brighter: 



7. Large circle much the brighter: 



8. Small circle much the brighter: 



9. Sides of box equal in brightness: 

 10. Circles equal in brightness: 



Explanation of abbreviations: S, small circle; L, large circle; 

 numbers, distances in centimeters of source lights from screen; 

 r, right; 1, left. • 



The experiment was undertaken with chicks no. 70 and no. 

 83. The preference tests showed a marked inclination of the 

 chicks to react positively to the larger or negatively to the 

 smaller of the two circles, each animal reacting 8 times to L 



