94 



HOMING AND RELATED ACTIVITIES OF BIRDS. 



was 12 seconds. Since it had made but one error during the whole course of 

 the experiment, and that on the third day of its preliminary trials, I think it 

 fair to call this place X = 7150, the red limit under the conditions of the present 

 test. The limit of D is evidently X = 7000, while that of R is between X = 7000 

 and X = 7150. When tested with light- waves of greater length, no response 

 was obtained. 



LIMITS AT THE VIOLET END. 



After a preliminary test upon green X = 5480, in which all choices were cor- 

 rect, the chicks were tested with violet X = 4500, with the Nernst as a source. 

 The stimulus was plainly visible to me. Six tests were given each animal, as 

 shown in table 27: 



TABLE 27. 



Again I became suspicous as to the adequate stimulus, especially in view of 

 Hess's work. In order to eliminate all diffuse and scattered white light I put 

 in a filter of CuS0 4 to which an excess of ammonia has been added immedi- 

 ately behind the selecting slit. The thickness of the layer was 1 cm. I first 

 tested with a pocket spectrometer, to see that only violet and bluish-violet 

 rays were admitted. It was necessary to open up the slits, Sj to 0.75 mm. 

 and S 2 to 1 mm., in order to make the light visible to me. The results are 

 given in table 28. 



In order to obtain fairly high intensities in the shorter wave-lengths I found 

 it necessary to substitute the automatic Bausch and Lomb arc lamp for the 

 Nernst.* The filter was arranged as before. The results appear in table 29. 



TABLE 28. 



TABLE 29. 



On the following day the arc was again used, but as an extra precaution 

 against white light I placed a filter of the same constitution as the above 

 (cupra-ammonium) in front of the slit in the collimator S^ The pocket 

 spectrometer showed that no light was admitted except the violet rays. 



The chicks were first tested with X = 3950. It was visible to me at the end of 

 a long adaptation time, but not visible under the conditions offered the chicks. 



*Yerkes and Watson, op. cit., p. 89. 



