42 HAROLD C. BINGHAM 



when the variable was made equal to the standard. The result 

 was a perfect series (see record 1 for November 25). 



With positive evidence that the discrimination was not of 

 size difference, an inventory of possibilities was taken. Among 

 the features noted was a small crack where the outside extension 

 of the electric source box jointed to the experiment box. But 

 a similar crack appeared on both sides in corresponding posi- 

 tions, so it seemed that in this factor there could be no cue by 

 which the chick had been guided in its choice of the proper 

 compartment. 



Closer inspection, however, proved that these two minute 

 cracks lay at the bottom of the matter. Where the rabbeted 

 edges of the shifter and tracks rubbed, there was a bright edge, 

 and wherever the shifter rested the bright surface was covered. 

 Thus when the shifter was at the left, the right end of the track 

 was uncovered, and from this uncovered part were reflected some 

 of the light rays from the upper illumination. A small portion 

 of this reflected light could enter the crack on the uncovered 

 side. Now, when the standard circle was presented at the left, 

 the shifter was always moved to the right; when the standard 

 stood at right, the shifter stood at the left. As a result, the 

 crack that was illuminated was always the one w^here the stand- 

 ard circle was displayed; the other crack, being reached by no 

 reflected light, was comparatively dark. 



The effect of closing these small cracks through which the 

 light was reflected is indicated in the records following the first 

 on November 25. The perfect reactions abruptly cease; out of 

 15 tests there are only 6 correct responses. As the following por- 

 tion of the table indicates, the experiment with the cracks 

 closed was continued, but the variable had to be reduced to 

 o 15+ before the correct reactions returned. 



The chick, then, had detected a faint streak of light not more 

 than 6 mm. long and less than 1 mm. wide or else the increase in 

 general illumination which resulted. By considering the 

 amount of light that came from the upper illumination, the poor 

 reflecting surface of a steel strap worn only moderately bright, 

 and the narrow surface from which the light was reflected, (not 

 more than 1 cm. in width), the insignificance of this strip of 

 light is emphasized. It was an example of the chick "outguess- 

 ing" the experimenter. 



