COLOR BLINDNESS OF CATS 123 



learn to discriminate what was for them close to a match. It was 

 not difficult to identify a complete confusion, for the number 

 of errors in each series almost invariably was twelve or more, 

 while for six hundred tests the errors were not far from fifty per 

 cent. On the other hand, the series in a difficult discrimination 

 would contain a smaller proportion of errors, but it was here 

 that greatest care was required. 



Let us summarize the precautions taken to guard against 

 discrimination by criteria other than the hue or intensity of the 

 paper. The glasses were kept scrupulously clean so as to show 

 no marks of any kind on their outer surface. They were all of 

 clear glass without flaws. All of the papers were cut by a single 

 pattern and placed in the glasses so that the overlapping edges 

 were on the opposite side from the cat and hence invisible to him. 

 Finally, circular disks of pasteboard were forced down within the 

 cylinders of paper so that they held the paper closely pressed 

 against the inner surface of the tumblers, and thus did away with 

 wrinkles or apparent differences of depth. Several observers 

 reported that the tumblers when thus prepared appeared to be 

 made of colored glass. The apparatus prevented looking into 

 the glasses before making a choice, and the experimenter put 

 the glasses in position and made all changes while the cats were 

 where they could not see the apparatus. 



To prevent choice by position, the glasses were presented in 

 random order, (one on the right, then on the left, then on the left 

 again and so on) , and they were changed in their positions along 

 the apparatus. Whenever a cat formed the habit of choosing 

 the glass on the left (or the right), the food-glass was placed no 

 the other side until the habit was broken up. 



To eliminate odor as a factor in discrimination, equal amounts 

 of food were placed in the food-glass and the confusion glass; 

 clean glasses and clean papers promptly replaced any that had 

 become soiled, and the papers were exchanged within the two 

 glasses so that the confusion-paper replaced the food-paper, and 

 vice versa. Aside from the odor thus taken into account, the 

 odor of the pigments of the two papers might presumably aid 

 discrimination, hence within the paper in the food-glass, we 

 placed a cylinder or lining of the confusion-paper and likewise 

 within the paper in the confusion-glass was placed a cylinder of 

 the food-paper. Moreover, the various papers were kept in one 



