STUDIES ON SPECTRAL SENSITIVITY OF BIRDS. 



91 



in H the tantalum light was turned out. Five seconds later (by metronome) 

 the door to the reaction chamber was opened. The chick then passed out and 

 reacted either positively or negatively to the light. When the animal had 

 immediately, or after errors, finally correctly responded (i. e. positively), as 

 shown by causing the proper light to wink (p. 90), the tantalum light was 

 again turned up. After feeding for 3 seconds the animals were allowed to 

 pass again into H through one of the side doors. The light was again turned 

 out for 5 seconds and the test repeated. At first only 6 to 8 tests were made. 

 The number was then increased to 10 and sometimes in control work to 1.5 to 

 20. In the following tests no effort was made to control the energy of the 

 stimulus. The light falling on the plaster surface was very intense — the max- 

 imum afforded by the spectrometer at any given wave-length. We planned 

 to test the animals first near the center of the .spectrum, X = 5480, and then to 

 pass on in the red region until a limit to response was reached ; next to re- 

 train on X = 5480, and then to pass to the violet region, continually decreasing 

 the wave-length until the limit was reached. Table 23 gives a summary of 

 the tests at X = 5480, showing that the retina of the chick is sensitive to 

 X = 5480, and furthermore, that the chick is distinctly positive to light. 



Table 23. 



LIMITS AT THE RED END. 



In order that no useless work might be done in testing the chicks at inter- 

 mediate points, I next tested them in a similar way at X = 6230; then at X = 6520 

 and X = 6825 (table 24). 



Table 24. 



As the animals were somewhat disturbed on the final test, due probably 

 to the low stimulating value of the light, I increased the intensity by opening 

 up the slit in the collimator S,, with the results shown in table 25. The 



