The Sense of Sight: Brightness Vision 101 



might choose, for example, not the white or the black box, 

 but the box which was to the right or to the left, in accord- 

 ance with its experience in the previous test. This would be 

 discrimination by position. As a matter of fact, the animals 

 have a strong tendency at first to go uniformly either to the 

 right or to the left entrance. This tendency will be exhibited 

 in the results of the tests. Again, discrimination might 

 depend upon the odors of the cardboards or upon slight 

 differences in their shape, texture, or position. Before con- 

 clusive evidence of brightness discrimination could be ob- 

 tained, all of these and other possibilities of discrimination 

 had to be eliminated by check tests. I shall describe the 

 various precautions taken in the experiments to guard against 

 errors in interpretation, in order to show the lengths to 

 which an experimenter may be driven in his search for 

 safely interpretable results. 



To exclude choice by position, the cardboards were moved 

 from one electric-box to the other. When the change was 

 made regularly, so that white was alternately on the right 

 and the left, the mouse soon learned to go alternately to the 

 right box and the left without stopping to notice the visual 

 factor. This was prevented by changing the position of the 

 cardboards irregularly. 



Discrimination by the odor, texture, shape, and position 

 of the cardboards was excluded by the use of different kinds 

 of cardboards, by changing the form and position of them in 

 check tests, and by coating them with shellac. 



The brightness vision tests described in this chapter were 

 made in a room which is lighted from the south only, with 

 the experiment box directed away from the windows. The 

 light from the windows shone upon the cardboards at the 

 entrances to the electric-boxes, not into the eyes of the mouse 

 as it approached them. Each mouse used in the experiments 



