LIGHT DISCRIMINATION IN THE ENGLISH SPARROW 87 



The lamps were occasionally interchanged to prevent the birds 

 from forming any habits due to difference in the quality of the 

 light which the observer was unable to detect. The discrimina- 

 tion box was frequently washed out as a precaution against the 

 bird getting any "cues" or reflections due to the waste material 

 which collected. 



The procedure for each test was as follows: The bird was 

 placed in the entrance chamber (C, Fig. 1) on the wet pad. 

 The light in the room was then turned off and the induction 

 coil started. After waiting several seconds to accustom the 

 experimenter's eyes to the darkness, the door Di was opened 

 and the bird allowed to pass into the discrimination box. (The 

 entrance chamber was covered with black velvet paper and 

 consequently was always dark. Hence the bird's eyes were 

 accustomed to the dark.) Having entered it the bird could 

 return to the entrance chamber only by passing through to the 

 right or to the left of the partition and then through either 

 door D 2 or D 5 and thence to C by way of the narrow alleys 

 C 2 or C 6 through C 3 or Co and through doors D. or D? into the 

 entrance chamber. After passing through the door D, the bird 

 nearly always hesitated several seconds and frequently several 

 minutes, before making a choice between going to one or the 

 other of the stimulus areas. If the bird chose correctly it was 

 allowed to pass on into the alleyway and back into the entrance 

 chamber. But if it made a wrong choice the circuit was closed 

 at the key and the bird shocked. The birds learned very soon 

 that when they received an electric shock they must go back 

 and around the partitions to the other side. In the earlier 

 experiments the birds would often stop just beyond the door D,. 

 In such cases the experimenter gave them instantaneous shocks 

 which caused them to move on and thus force them to discrim- 

 nate. But after they learned what they were expected to do 

 and discrimination became established, they would hesitate only 

 a few seconds before making a choice. In the later experiments 

 it was seldom necessary to shock them except when they made 

 a wrong choice. 



A series of correct choices for two consecutive days — 30 trials 

 — was counted as correct discrimination and the difference in 

 intensity of the standard and the variable light was decreased. 

 At the beginning of the investigation the standard light was 



