THE COLOR VISION OF BIRDS 11 



down to 1/90 of the standard; at this energy both are above 

 the chick's light adapted threshold. Chick A, fully light adapted 

 was placed in the experiment box. He chose — 



Green Yellow 



7 7 



Chick C was dark adapted for one hour, then offered the lights. 



He chose — 



Green Yellow 



7 5 



Light adapted immediately afterward, he chose — 



Green Yellow 



4 5 



At this intensity neither of the colors has a markedly greater 

 stimulating value than the other. At the same energy the red 

 drops below the light adapted threshold. 



A point was determined at which the yellow was just brighter 

 than the green for the light adapted chick. This was yellow 

 4/90, green 1/90. Light adapted, Chick A chose — 



Green Yellow 



3 21 



After dark adaptation he chose- 



Green Yellow 



4 2 



Chick C was tested in the same way but for some unexplained 

 reason showed a complete breaking down of his reactions; even 

 failing to choose between light and darkness. This lasted for 

 several days and Chick A also became erratic so that the tests 

 could not be continued without retraining for brightness. Such 

 experiments, with food at every trial, make heavy demands 

 upon the retention of earlier habits and it is surprising that 

 the reaction persisted as well as it did. The apparent shift 

 in brightness for the yellow and green, like the last test recorded 

 for the red and green, indicates a Purkinje effect of adaptation. 

 The determination of energies of the monochromatic lights 

 having equal brightness values for the chick (the brightness 

 equation point) is of interest when compared with the respective 

 thresholds for the same wave-lengths and is of primary im- 

 portance for tests for color discrimination. The values that I 



