PATTERN-DISCRIMINATION IN VERTEBRATES 217 



spectrum, and are highly corrected for spherical aberration, 

 astigmatism, distortion and curvature of the image. With this 

 arrangement light from all parts of the test-field emerged from 

 the lens in bundles of parallel rays. The eyes of dog 1 being 

 emmetropic, it follows from geometrical optics that if his un- 

 accommodated eye were placed in any part of the cone of light 

 emerging from the lens, a sharp image of the test-field would be 

 formed on the retina, and that the angle subtended by the image 

 of any stripe at the second nodal point of the eye would be the 

 same as that subtended by the stripe itself at the first nodal 

 point of the projection lens. 



The stripes on the positive field were horizontal, and each one 

 was 3.902 mm. wide. The distance between the test-field and the 

 first nodal point of the lens was 399 mm. 13 The image of each 

 stripe therefore subtended a visual angle of 33' 32" — an angle 

 larger than is subtended by the sun's disc at the earth. The pat- 

 tern on the positive field is closely comparable with that of a plank 

 fence, the units in which are 6 inches wide and 6 inches apart, 

 viewed at a distance of 50 feet. The stripes on the negative field 

 were also horizontal, but were only 0.1 mm. wide. They were re- 

 solved by the lens, and may have been resolved by the dog's eye, 

 but being only 1-39 as wide as those on the positive field, an 

 animal capable of distinguishing them as such should have had no 

 difficulty in discriminating between them and the coarser system. 



I introduced the dog to this problem 22 September 1915, 

 using the general method already described. The mean bright- 

 ness of both fields was 6.2 candles per square meter — a com- 

 fortable reading condition for the light adapted human eye. 

 The intensity of the electric shock administered for incorrect 

 responses was the same as that used in the immediately preceding 

 experiment, in which it had proved effective and not greatly 

 disturbing. The results of this experiment are shown in table 2. 

 They show that the animal did not discriminate and did not 

 improve in 500 trials. Since this dog was already trained to 

 react in the box, and since he had learned readily to discriminate 

 between other purely visual stimuli, I regard the evidence as 

 conclusive that the dog does not distinguish relatively gross 

 detail in visual objects, and that the deficiency is retinal. 



13 1 wish to thank Dr. A. G. Worthing and Dr. W. Weniger for making this deter- 

 mination for me. 



