THE COLOR VISION OF BIRDS 21 



and seems to be surrounded by a luminous halo. It seemed 

 possible that the birds were reacting upon the basis of the 

 sharpness of the visual objects rather than upon their color, 

 either as a result of diffraction or of serious chromatic aberration. 

 Tests were made at different times during the experiments 

 with a view to controling these factors and gave the following 

 results. 



1. Wide variations in the size and shape of the stimulus plates 

 were without effect. 



2. High intensities and dilutions, reducing the apparent 

 diffraction of the shorter wave-lengths and giving sharp out- 

 lines for me to both stimulus plates did not effect the reaction 

 of the birds. 



3. Intense white light with imperfect color filters, completely 

 altering the secondary phenomena for me, were discriminated 

 as promptly as the spectral lights. 



4. The evidence given in the following section for abrupt 

 changes in the reaction value of the spectrum is difficult to 

 explain by these phenomena. 



The visual acuity of the fowl determined by Johnson ('14) 

 seems to eliminate the question of any great chromatic aberration 

 in the chick, and the foregoing tests make a reaction upon any 

 other basis than retinal sensitivity highly improbable. 



THE CHARACTER OF THE SPECTRUM 



With the establishment of sensitivity to differences of wave- 

 length the problem of the difference limen for wave-length in 

 different parts of the spectrum at once suggests itself. A 

 thorough solution of this problem would require months of 

 training with careful controls of brightness at every stage. The 

 Yerkes- Watson apparatus is not well adapted for such exper- 

 iments as it cannot be arranged readily to give lights from 

 adjacent parts of the spectrum. The following tests, serve to 

 give a clue to the form of the spectrum of the bird, particularly 

 with reference to the existence of regions of widely different 

 reaction value separated by relatively short intervals of the 

 spectrum. 



Chick A, positive to 650 pp. and negative to 520 pp. was 

 offered red 650 and yellow 580 pp. He chose the red six times 

 in seven trials and showed little confusion. The red was evi- 



