98 



HOMING AND RELATED ACTIVITIES OF BIRDS. 



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

 able and ]3roportional to the absolute energy required for stimulation. 



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

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

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

 clo.sely 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 ilid not enable us to make 

 tests with wave-lengths shorter than X = 4500. The decrease 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 e 

 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 liecn taken and the absolute thresli- 

 old hail lieen desired, there is little room for 

 doul>t that the curve for the chick would 

 more ncarlj- coincide with that of man. I 

 draw this conclusion from numerous chance 

 exj)eriments, .some of which are cited in the 

 text. A separate investigation for each 

 wave-length would be required to thor- '''" '^" '*" ^* ■'-'' ^™ J'-'ji""» 



OUghlv WOlk out the absolute threshold ^''^=- lO-Sonsitivity Curve (dark- 



, • . , rifl.apted eve). A, chick; is, niau. 



values tor chick and man. At X = 6()()0 tlie The ordinate shows the reciprocal 



present curve shows that the threshold of d ">"'■ = OD of energy value 



,■ 1.- -i • .1 i-i4i ■ nece.ss.arv to produce reaction at 



stnnulation is lower m the chick than in ,ho wavo-iensth given on the al^ 



RESUME OF RESULTS OF STUDY ON CHICK. 



(1) The limit of the chick's spectrum at the red end lies probai)ly between 

 X = 7000 and X = 71.30; at the violet end lietween X = 3950 and X = 40.')0. The 

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

 These limits arc flexil)le and depend somewhat on the absolute intensity of 

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

 visual responses are completelj' lacking. I'nlcss the retinas of other day birds 

 show sensitivity in the red far beyond the ]5oint at which response ceases in the 

 chick, any theory to account for homing such ;\s that of Duchatel remains 

 without scientific support. 



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

 of (lifl'erent i-egions arc similar thi-oughoiit their cour.se. except in the extreme 

 red. 



*See the work and historical summary of A. Pfluger, Annalen der Physik, 9 (1902), pp. 18.5-208. 

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

 In most of Pflilger's work there is evidence of a secondary maximum. While 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. 



