VISUAL PERCEPTION OF THE CHICK 53 



On the basis of choices and behavior — -it seems safest to ex- 

 clude the time factor — it appears that the chick's hmit in dis- 

 crimination from a standard o 28-f Hes somewhere between 

 o 15+ and o 19 + . To express the relation in terms of centi- 

 meters instead of square centimeters, the size in diameter of the 

 variable is above 4.5 but below 5 ; that of the standard is 6. 

 The threshold of difference with a standard 6, therefore, is one- 

 fourth to one-sixth. 



To be certain that the reactions of the chicks were not made 

 on the basis of some detail or complex other than size difference, 

 it was necessary to employ various controls. There appear, in. 

 the stimuli themselves, three factors other than size that might 

 provide a basis for correct response: (1) luminous intensity of 

 the respective stimuli; (2) brightness of the respective stimuH; 

 and (3) illumination of the respective electric compartments. 

 Without adequate controls, it would be impossible to say definitely 

 whether size or these other characteristics were the bases of the 

 responses which point to the above conclusion. 



Supposing the two stimulus areas to be 6 cm. and 3 cm. in 

 diameter, the display surfaces equal in brightness, and a com- 

 plete image of the larger formed on the retina, it follows that 

 the chick would be stimulated by four times as much light from 

 the larger as from the smaller stimulus. It is conceivable that 

 the chick, under these conditions, might respond on the basis 

 of pain or its physiological concomitant, or even of pupillary 

 reaction. In addition to this possibility, the general illumina- 

 tion of the electric compartment containing the larger stimulus 

 would differ from that of the other compartment. The chick 

 might choose the lighter or darker compartment. Both of 

 these possibilities might occur independently of the matter of 

 brightness. 



As a portion of the controls used in this connection, an addi- 

 tional lamp was placed directly above the experiment box. 

 This upper illumination (L of figure 3, p. 24), consisting of a 

 2 c.p. electric lamp enclosed in a vertically suspended cylinder 

 of galvanized iron, was designed to hide the inequalities in the 

 distribution of light in each compartment. It hangs about 120 

 cm. above the floor of the experiment box so that an equal 

 amount of light is added to each compartment. The diameter 

 of the cylindrical enclosure is 10 cm. and its length is 20 cm. 



