28 HAROLD C. BINGHAM 



3. Stimulus shifter 



Figure 6 also presents a general view of the stimulus shifter, 

 the primary function of which is control of the various visual 

 details. The stimulus shifter or adapter is also described in 

 Behavior Monographs along with the description of the source 

 box. 



A considerable number of stimulus plates, used for varying the 

 size and form of the stimuli, is required for determining the 

 chick's threshold for these spatial details. The majority of the 

 plates used in this study is included in table 1. Owing to varia- 

 tions in sensitiveness among different subjects, a set which meets 

 the requirements for one animal does not suffice for another, 

 hence, in those cases where the size difference among the plates 

 varies by one millimeter, a safe margin has been allowed by 

 enumerating a few more plates than would ordinarily be 

 required. 



4. Accessories 



In my report from which figures 3-5 are reprinted, there 

 appears, on page 73 and following, a description of the details 

 designed to aid in the control and manipulation of the mechan- 

 ism just described. The punishing device and the upper illu- 

 mination are also described. It is there shown how the experi- 

 menter from his position for observation, can control by means 

 of ropes, pulleys, and weights the entire apparatus. 



