STUDIES ON SPECTRAL SENSITIVITY OF BIRDS. 103 



The position habit has been corrected and the animal is once more automatic 

 in its responses. 



January 27. X = 7120. Arc lamp; other conditions as on January 24. In to-day's series 

 instead of again trying animal on X = 6840, at which point a position habit began to appear, 

 I gave light somewhat further along, X = 7120. 



After adjusting the intensity of the light so that I could see it (10 minutes adaptation) 

 20 seconds after tungsten light was turned off, the following results were obtained; 100 per 

 cent correct: 



Light left, response right. 

 Light right, response right. 

 Light left, response right. 



Light right, response right. 

 Light left, response right. 



In the above I had the slit in the collimator at 0.75 mm., which is too wide 

 of course to give a very "pure" spectrum. I next narrowed this slit to 0.5 mm. 



Light right, response right. 

 Light left, response wrong. 

 Light left, response right. 



In the three trials last considered the animal began to wait from 1 to 3 minutes 

 to respond. Fearing another breakdown, I again opened the slit to 0.75 mm. 



Light right, response right. 

 Light left, response right. 

 Light right, response right. 



Light right, response right. 

 Light left, response right. 

 Light left, response right. 



Light right, response wrong. 



There can be but little question of sensitivity here when the intensity is 

 great. The breakdown at X = 6848 was unquestionably due to the position 

 habit. In the present series, the slit is so wide that it can not be determined 

 whether the animal is reacting to the wave-length 7120 or to those slightly 

 shorter which are undoubtedly present in the beam. It is hardly worth while 

 to try to determine the limits any more accurately. The extreme red rays have 

 little stimulating effect and the slits have to be opened wide in order to obtain a 

 ray of sufficient intensity to call out response. Under such conditions we can 

 no longer guarantee the purity of the light. One crucial test remains, however, 

 viz, to give the animal the opportunity to react to rays beyond the (human) 

 visible spectrum. 



January 28. X = 8000. Arc lamp as source. Collimator slit opened to 1 mm. and slit 

 in objective to 1.25. The wave admitted ranged probably from 7700 to 8100. That the 

 infra-red rays were falling upon the plaster surface was determined by the use of the selenium 

 cell. No light was visible on plaster surface to me (or to Dr. Lashley) even after long 

 darkness adaptation. Large deflections in the galvanometer were noticed whenever the 

 selenium cell was brought in immediately above the plaster surface. One series of 10 trials 

 was given. The order of the series was determined in advance by the throwing ot a die. 

 The results were as follows: 



Light right, response wrong. 

 Light left, response right. 

 Light right, response wrong. 

 Light right, response wrong. 

 Light left, response right. 



Light right, response wrong. 

 Light right, response wrong. 

 Light right, response wrong. 

 Light left, response right. 

 Light left, response right. 



Again we see the onset of a pronounced position habit. The animal went 

 to left side on every trial, regardless of position of light. 

 I then gave the animal immediately X = 5700. 



Light right, response wrong. 



