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 uncjuestionably 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 tiiat, since 

 the chicks were already ti'ained by this work on the extremes of the spectrum 

 to respond to faint stimulation by light, it would be advisable to extentl 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-adajDted eye, and 

 f(ii- c>'es possessing various abnormalities in sensitivity to monochromatic 

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

 physical controls. There is general agreement that the maximal sensitivity 

 for the light-adapted eye lies in the I'egion of the yellow-green, and for the 

 dark-adapted eye in the bluish-green. 



Furl In'i'niore, it is de.sirable to test the ciucstion whethei' the course of such a 

 curve, V. lien obtained in the chick, will ho lik(> that found in man. Comparison 

 can be made only when tlie data for the two curves have liccn nl)tained under 

 similar conditions. 



Our ])rocedure in obtaining the curves shown below was as follows: We 

 fii'st obtained refinement of the monochromatic light by the use of the auxiliary 

 spectrometer (p. 88). Each monochromatic bundle licfoi'c 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 tiie Brodhun sector. Since the dilTerent bimdles worked with all 

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

 l^lotted throughout the spectrum directly from the angular oi)ening of the 

 sector just sufficient to jiroduce a response at the given «'ave-length, or as is 

 more usual from the reciprocals of these values. In tlie cui've these sector 

 values are averaged for the three chicks. 



The method of obtaining the chick's respon.se was similar to that used in 

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

 The motor driving the Brodhun .sector was started and 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 ailaptation, then the tantalum light was turned up fni- 5 seconds, then 

 came a wait of 5 seconds in complete darkness. The tUntv to the reaction 

 chamber was next raised and the chicks were allowed to lesi^ond. Tlu; stimu- 

 lus on (he first trial was always well aliove the threshold. On the second trial 

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

 t rials, s( ill further decreased, until at last a break-down occiured, /. c, the chick 

 refused to respond, or else waited long |)ast the usual time for response, for 

 ;i<l.ipta,tion to increase the stimulating effect of the light. I tried to regulate 



