Yerkes, Modifiahility of Behavior. 263 



we are forced to ask, Do the jDlasticity experiments reveal anything 

 except age differences with respect to what might be termed the 

 educability of light vision? With the hope of getting further light 

 on this problem, I carried out additional experiments, with the indi- 

 viduals used in the Weber's law apparatus, by a method whose form 

 and results will now be described. 



3. Experiments with one side of discrimination hox covered in 

 varying degrees. For this work the cardboards were removed from 

 the discrimination box which had served for the plasticity experi- 

 ments, and difference in the illumination of the two boxes was ob- 

 tained by covering, with a jDiece of black cardboard, the whole or a 

 part of the top of one of the two small boxes. The total inside length 

 of the boxes was 29 cm. I have described the condition of the darker 

 box by giving in terms of a fraction the amount of the top which was 

 covered. Thus 18/29 means that the cardboard covered 18 of the 

 29 cm., beginning at the entrance and extending toward the rear 

 of the box. Shifting the lighter box (the one to be chosen) from 

 side to side involved merely the moving of the black cardboard 

 from the top of one box to the top of the other. 



After the experiments just reported had been completed, mice 

 jSTos. 170, 95, 294, and 293 were given training tests in the dis- 

 crimination box under the above conditions. Table 13 presents 

 the condition of discrimination as well as the results of the various 

 series of tests. When, as at the outset, the whole of one box was 

 covered, discrimination was extremely easy, because the boxes dif- 

 fered greatly in illumination. 



From the first, as the data of Table 13 indicate, the young 

 animals learned more rapidly than did the old ones. We have in 

 these results, therefore, additional support for the belief that dis- 

 criminating ability is more readily gained by the young dancer. 



It may not be out of place to remark here that the simple form of 

 the lighter-darker discrimination apparatus which served for this 

 series of experiments is precisely what should have been used 

 throughout this investigation. It has taken me years to learn that 

 it is not only possible, but also perfectly easy, to devise a con- 

 dition of experimentation which should be readily and accurately 



