76 HAROLD C. BINGHAM 



is j S.j 744 sq. cm.), and to reject o 7+, unequal amounts of 

 light from the stimulus sources will be admitted to the electric 

 compartments. If the intensities of the two stimulus areas be 

 equalized, the compartment at the end of which the larger 

 circle appears will be the more highly illuminated. Without 

 the elimination of this factor of general illumination, the ex- 

 perimenter can not be certain that his animals are discriminat- 

 ing on the basis of size ; they may be choosing the lighter com- 

 partment. To overcome this difficulty the upper illumination 

 was used and the elongation of the electric compartments, 

 OOO, figure 2, was introduced. The amount of upper illumi- 

 nation should therefore depend upon the degree of difference 

 between the two stimulus areas, and in a quantitative study, 

 where the difference is continually being reduced toward the 

 minimum of the animal's discriminative ability, the difficulty 

 practically disappears. 



A further value of the upper illumination is economy of time 

 for the experimenter. It aids in emphasizing, from the first, 

 the visual factor which is being tested and thus prevents the 

 animal from acquiring a habit of discrimination which later 

 must be inhibited and replaced by a different habit. After an 

 animal once acquires a definite way of reacting to stimuli, no 

 little time and pains are required to make it change its mode 

 of response. 



With the elongation of the electric compartments appears 

 another difficulty differing somewhat from the matter of general 

 illumination. This is the problem of the chick's optimal focal 

 distance. It is quite possible that, for a stationary stimulus, 

 the discrimination chamber of this apparatus is too far away. 

 The anatomy of the eye as well as the behavior of the animal 

 should be considered in connection with this problem. The 

 matter of distance perception, however, falls without the scope 

 of the present task. 



II. PROBLEM, METHOD, AND TECHNIQUE 



i. The chicks 



The experiments reported in this paper were made with 

 about 25 chicks belonging to three different groups. The first 

 group, originally consisting of 10 chicks, was secured from a 



