98 



HOMING AND RELATED ACTIVITIES OF BIRDS. 



opening at which the various thresholds were obtained are all directly compar- 

 able and proportional to the absolute energy required for stimulation. 



The relative stimulating effect of the different regions of the spectrum for 

 the (partialljO dark-adapted eye appears clearly in the curve. The maximum 

 efficiency for both chick and man is found at X = 5300, which agrees fairly 

 closely with what previous work we have upon man.* From the maximal 

 point one finds a decrease in efficiency in passing toward either end. It is 

 unfortunate that the calibrating of the selenium cell did not enable us to make 

 tests with wave-lengths shorter than X = 4500. The deci'ease in efficiency is 

 enormous as one reaches the red. While in every region except red the curve 

 of the human lies higher above the base-line 

 than that of the birds, it is questionable ? 

 whether the human has a lower absolute 

 threshold than the chick. The method of 

 obtaining the threshold of response for the 

 chick here used was a rapid one. If more 

 care had been taken and the absolute thresh- 

 old had been desired, there is little room for 

 doubt that the curve for the chick would 

 more nearly coincide with that of man. I 

 draw this conclusion from nimierous chance 

 experiments, some of which are citerl in tlie 

 text. A separate investigation for each 

 wave-length would be required to thor- 

 oughly ^\ork out the absolute threshold 

 values for chick and man. At X = t)600 the 

 present curve shows that the threshold of 

 stimulation is lower in the chick than in 



Or.O Wi ^0 550 OJO 



Fig. 10. — Sensitivity Curve (dark- 

 adapted eye). A, chick; B. man. 

 The ordinate shows the reciprocal 

 (1 mm. = .01) of energy value 

 necessary to produce reaction at 

 the wave-length given on the air- 



man. 



RESUME OF RESULTS OF STUDY ON CHICK. 



(1) The limit of the chick's sjwctrum at the red end lies probably Ijetween 

 \ = 7000 and X = 7150; at the violet end between X = 3950 and X = 4050. ^The 

 limits for the human eye have been found to lie between X = 4000 and X = 7000. 

 These limits are flexible and depend somewhat on the absolute intensity of 

 the light and necessarily upon the state of adaptation. Beyond these limits, 

 visual responses are completely lacking. Unless the retinas of other day birds 

 show sensitivity in the red far beyond the point at which response ceases in the 

 chick, any theory to account for homing such as that of Duchatel remains 

 without scientific support. 



(2) The curves for man and chick showing the relative stimulating effect 

 of different regions are similai- throughout their course, except in the extreme 

 red. 



*See the work and historical summary of A. Pflilger, Annalen der Phyaik, 9 (1902), pp. 185-208. 

 As will be seen, the curve obtained from the human subject here used shows only one maximum. 

 In most of Pfluger's work there is evidence of a second.ory maximum. WhUe his work was 

 apparently much more carefully controlled than my own, I am convinced that the secondary 

 maximum obtained by him is due to some failure properly to control the energy of the stimulus 



