56 HAROLD C. BINGHAM 



had not been completed. If the previous correct reactions were 

 not noticeably interrupted by this control system, the chick was 

 considered ready for a larger variable. With the exception of 

 the records for chicks 3,6, and 7 the essentials of this procedure 

 were carried out for every record given in table 3. The results 

 presented in tabular form, therefore, are those of controls; they 

 are not training or learning records. 



The significance of having the + compartment darker than 

 the —compartment in most of the tests of this control series 

 appears when the plan of the preliminary training is set forth. 

 In the beginning, the chick was required to choose on the basis 

 of a visual complex consisting of (1) the lighter compartment in 

 which the stimulus area was (2) larger, (3) triangular, and (4) 

 brighter. As soon as consistent choices appeared, the inequali- 

 ties between the two stimuli, except size difference, were 

 gradually removed by replacing the triangle with a circle, in- 

 creasing the upper illumination, and gradually bringing the 

 source lamps to an equal distance from the diffusing or display 

 surfaces. Next, the source distances were gradually and irreg- 

 ularly made unequal. Possibility of discriminating on the 

 basis of brightness and luminous intensity, or either, was thus 

 eliminated before reaching the final control stage. In the more 

 thoroughgoing control series, therefore, the most important 

 fact to establish was certainty about the possibility of general 

 illumination. The + compartment was thus made darker after 

 the first two tests while the other variations were continued. 

 In the matter of general illumination, then, the conditions of 

 the final controls were exactly reversed from those of the pre- 

 ceding training series. 



This method of preliminary training was adopted after the 

 perplexing experience with chick 3, described in Chapter IV, 

 and the discovery of the crack which admitted reflected light. 

 The incident suggested that the chick which was being trained, 

 preliminary to the primary threshold study, should be aided in 

 this early stage by having the differences between the two visual 

 stimuli emphasized by a combination of light factors.^ Two 

 stimulus areas, differing from each other with respect to one 

 quality only, present to the chick a problem which is quite 



^Yerkes, R. M. The discrimination method. Journal Animal Behavior, 

 vol. 2, p. 142. 



