PATTERN-DISCRIiMINATION IN VERTEBRATES 207 



animal was accidentally killed. Meantime I had acquired no 

 information regarding the refraction of his eyes. Consequently 

 I did not feel justified in publishing the report of the work as 

 containing anything decisive or important. It is of some in- 

 terest when considered in connection with some later experiments 

 described below. 



Three years ago I became interested in the question of the 

 relative visual sensitivity of animals possessing different types 

 of retinal structure. The retina of the dog's eye is fairly typical 

 of the mammals below the primates. There is no fovea, 4 rather 

 indefinite "sensitive areas" and numerous opaque nerve-fibers 

 which must be considered as blind spots. Such a retinal struc- 

 ture is characteristic, in the main, of mammals below the pri- 

 mates. It seemed fair to select the dog as a representative of 

 this class. I chose the domestic chicken as representative of 

 animals possessing a definite sensitive area but no fovea, and 

 the Capuchin monkey as representative of animals possessing a 

 fovea centralis, temporal to the entrance of the optic nerve. 

 I also felt impressed by the apparent superiority of stimuli 

 such as Casteel 5 used, over stimuli differing only in outline, for 

 a test of the question in which I was particularly interested. 

 I selected 6 as the stimulus to be chosen by the animal, a cir- 

 cular field, 6 cm. in diameter, covered by a system of horizontal 

 dark and bright stripes of equal width. The number of stripes 

 on the field and the width of each stripe could be varied by 

 insensible gradations. The field to be rejected was covered by 

 a similar system of striae, but the members were so numerous 

 and so fine that the eye would not resolve them at the minimal 

 distance at which the animal could make his choice. The stim- 

 uli differed in no other respect. The sensitivity of the animal 

 was to be measured by the angle subtended at the eye by the 

 separation of the upper edge of one bright stripe from the lower 

 edge of the bright stripe just above it. The reader should con- 

 sult the original report for details of these tests. 



The daily results obtained for Dog 1, a pure-bred English 



4 Slonaker, J. R. A comparative study of the area of acute vision in verte- 

 brates. Journal of Morphology, vol. 13, no. 3, 1897. 



5 Casteel, D. B. Discriminative ability of the painted turtle. This journal, 

 vol. 1. 1911, pp. 1 ff. 



6 Johnson, H. M. Visual pattern-discrimination in the vertebrates. I and 

 II. This journal, vol. 4, 1914, pp. 319-361. 



