96 HOMING AND RELATED ACTIVITIES OF BIRDS. 



one. In my experiments, where a large stimulating surface was used, the ques- 

 tion of visual acuity does not enter in. It does unquestionably in their work. 

 Furthermore, their work requires a reaction which runs counter to the chicks' 

 ordinary habits. The birds in daily life constantly have occasion to pick up 

 greenish-yellow grains, yellow grains, and reddish-yellow grains, but rarely 

 or never blue and violet grains. 



RELATIVE STIMULATING EFFECT OF MONOCHROMATIC LIGHT. 



While at work upon the limits of the spectrum, it occurred to me that, since 

 the chicks were already trained by this work on the extremes of the spectrum 

 to respond to faint stimulation by light, it would be advisable to extend the 

 scope of the experiment so as to obtain the curve of the relative stimulating 

 effect of light of different wave-lengths. Such a curve has never been ob- 

 tained hitherto on animals. It has been worked out for man under several 

 sets of conditions — for the light-adapted eye, for the dark-adapted eye, and 

 for eyes possessing various abnormalities in sensitivity to monochromatic 

 light. This work, even on man, has been done in general with very poor 

 physical controls. There is general agreement that the maximal sensitivity 

 for the light-adapted eye lies in the region of the yellow-green, and for the 

 dark-adapted eye in the bluish-green. 



Furthermore, it is desirable to test the question whether the course of such a 

 curve, when obtained in the chick, will be like that found in man. Comparison 

 can be made only when the data for the two curves have been obtained under 

 similar conditions. 



Our procedure in obtaining the curves shown below was as follows: We 

 first obtained refinement of the monochromatic light by the use of the auxiliary 

 spectrometer (p. 88). Each monochromatic buntlle before being admitted to 

 the dark room was made to bear a standard amount of energy. The thresholds 

 for the animals were then obtained by cutting down the energy of the light 

 by means of the Brodhun sector. Since the different bundles worked with all 

 have the same initial energy when admitted, the .sensitivity curve can be 

 plotted throughout the spectrum directly from the angular opening of the 

 sector just sufficient to produce a response at the given wave-length, or as is 

 more usual from the reciprocals of these values. In the curve these sector 

 values are averaged for the three chicks. 



The method of obtaining the chick's response was similar to that used in 

 the work on limits. The apparatus was set for work at a given wave-length. 

 The motor driving the Brodhun sector was started antl left running during the 

 whole course of the experiment. The intensity of the light falling on the 

 stimulus patch was at first high (full 140 mm. of the Brodhun sector, but with 

 the smoked glass in place). The chicks were given 10 minutes' complete 

 darkness adaptation, then the tantalum light was turned up for 5 seconds, then 

 came a wait of 5 seconds in comjilete tlarkness. The door to the reaction 

 chamber was next raised and the chicks were allowed to respond. The stimu- 

 lus on the first trial was always well above the threshold. On the second trial 

 the angular opening was made considerably smaller; and on the succeeding 

 trials, still further decreased, until at last a break-down occurred, i. e., the chick 

 refused to respond, or else waited long past the usual time for response, for 

 adaptation to increase the stimulating effect of the light. I tried to regulate 



