The Sense of Sight : Brightness Vision 1 1 5 



another in brightness by a considerable amount were chosen 

 from the Nendel series; these were numbers 10 and 20. It 

 seemed certain, from the results of previous experiments, that 

 the discrimination of these papers by brightness difference 

 would be possible, and that therefore the use of papers be- 

 tween these two extremes would suffice to demonstrate the 



FIGURE 16. Three of Nendel's gray papers: Nos. 10, 15, and 20. To exhibit 

 differences in brightness. 



delicacy of discrimination. In Figure 16 we have a fairly 

 accurate representation of the relative brightness of the 

 Nendel papers Nos. 10, 15, and 20. 



Pieces of the gray papers were pasted upon cardboard 

 carriers so that they might be placed in the discrimination box 

 as were the white and black cardboards in the tests of bright- 

 ness vision previously described. Mice which had been 

 trained to choose the white box by series of white-black tests 

 were now tested with light gray (No. 10) and dark gray 

 (No. 20), my assumption being that they would immediately 



