REACTIONS OF CHICKS TO OPTICAL STIMULI 285 



of the sliding frame, each composed of two plates of red glass 

 cut from the same sheet; one blue screen, in the middle of the 

 frame, made up of three plates of blue and violet glass. These 

 color names represent simply what the colors appeared to the 

 experimenter to be in advance of spectroscopic analysis. In 

 each of the three screens the colored plates were covered on the 

 side toward the chicks with a plate of opal glass, and over this 

 was set a black mat with a circular opening 8 cm. in diameter. 

 A spectroscopic analysis of the light transmitted by these screens 

 was kindly made for the writer by Professors Yerkes and Cole, 

 which showed the range of red to be from 760 to 640^/x, and 

 that of the blue-violet from 480 to 430/z/z. 



That young chicks are positively phototropic was suggested by 

 their crowding toward the light shortly after hatching in the 

 incubator. The same tendency was indicated also by prelim- 

 inary tests 4 and 5 (see table 1), in which no. 65 markedly 

 preferred the brighter of two non-chromatic stimuli. When red 

 and blue were approximately equal in brightness for the human 

 eye, red was preferred by the chick. See preliminary tests 1-3. 

 Even when red was considerably less bright than blue, as judged 

 by the human standard, the chicks still selected red in prefer- 

 ence to blue. After no. 65 had selected the dim red in series 

 6 in preference to the comparatively bright blue, series 8 

 and 9 were arranged to discover whether the animal was exhibit- 

 ing either a negative reaction to blue or a positive reaction 

 to darkness. The result showed clearly that neither of these 

 possibilities obtained. Blue 10 was preferred to darkness, 

 9-1. But when the distance of the source light from the 

 color screen was increased from 10 to 80 cm., the prefer- 

 ence of the chick for blue as against darkness was only slightly 

 evident. Again, when the stimuli were red 100 and blue 10, 

 no. 58 and no. 65 each selected blue nine times, red once. When, 

 with no. 58, blue was kept constant at this intensity and red 

 was gradually increased in brightness to 40, the preference was 

 as gradually shifted to red. A like result was found in the tests 

 on no. 65 immediately following. Thus, while the chicks had 

 a natural preference for the brighter of two non-chromatic 

 stimuli, the same could not be said of chromatic stimuli as 

 humanly estimated. The peculiar preference for red when 

 opposed to blue of much greater brightness value pointed to 



