THE COLOR VISION OF BIRDS 9 



the greater stimulating value of the green is lost, irrespective 

 of the degree of adaptation. 



The results are thus far valuable only as illustrating sources 

 of error in tests for the Purkinje phenomenon. In tests a and d 

 no evidence for a greater stimulating effect of either light was 

 obtained; in b and c the results are due, in all probability, to 

 the reduction of the intensity of the red to threshold value 

 and not to the process of adaptation. It seems probable that 

 two factors may produce spurious evidence of the Purkinje 

 phenomenon in experiments with animals. 1 : While two lights 

 of unequal energy may not produce a reaction upon the basis 

 of brightness at high intensity, a reduction of their intensity 

 may bring the weaker below threshold while the other is still 

 visible. From data obtained during training with white lights 

 it is clear that when two lights differing greatly in stimulating 

 value are exposed together the reaction to the less intense may 

 be lost while it still persists when this stimulus is exposed 

 alone or with another of equal stimulating value. In work 

 of this sort it seems necessary, then, to distinguish two types 

 of threshold; a lower threshold determined by the absolute 

 sensitivity of the eye, and a higher one, which might be termed 

 the threshold of attention, due to the interference of the stim- 

 uli. 2: With lights of unequal intensity differing in brightness 

 at high intensity by less than the limen of the animal the action 

 of the Weber-Fechner law might produce a perceptible differ- 

 ence following a reduction of the intensity of the lights. 



These factors, in all probability, would have produced the 

 ♦results obtained in all the recorded tests for the Purkinje phe- 

 nomenon if a band of white light of properly graded intensity 

 had been used instead of the spectrum. I am inclined to think 

 •that the methods of Hess, Katz and Revesz, Yerkes, and my 

 own recorded above are too crude to give a proof of the Purkinje 

 effect and that the results obtained are due rather to one or 

 other of these secondary phenomena. The records of the Pur- 

 kinje effect in man are so conflicting that an analogy between 

 man and other animals seems hardly profitable. The resem- 

 blance in the distribution of cones of the day bird's retina to 

 the human fovea, where probably the phenomenon does not 

 appear (Nagel '11), is sufficient, however, to demand extreme 

 caution in interpreting results in this field. 



