220 H. M. JOHNSON 



ference in wave-length does not necessarily indicate that its 

 possession is useful to the animal, unless there is evidence that 

 his eye will "resolve" the patterns. Leaving the question of 

 color-vision open, it should be remarked that the dog's retina 

 seems well adapted to sensitivity to time-rate of change in in- 

 tensity. In uncontrolled behavior all the normal dogs with 

 which I have worked attended instantly to moving objects. I 

 have often seen dogs chase the moving shadow of a cloud across 

 the field, and bark at it vigorously. Sensitivity to rate of flicker, 

 of various kinds and at various intensities, in lower mammals, 

 constitutes a problem of great interest and importance. There 

 is good ground for suspecting that such visual sensitivity is the 

 most useful one which the lower mammals possess. 



I wish to take this opportunity to acknowledge my indebted- 

 ness to Dr. P. W. Cobb for the work of examining the eyes of 

 these animals; also to him and to Professors S. O. Mast and 

 Harvey Carr for criticism which affected the direction and extent 

 of the work. Finally, I should like to thank Mr. Arthur Law- 

 rence, the owner of dog 1, for his generosity in permitting me 

 to keep the animal for several months. It might have been 

 quite difficult to find another dog with emmetropic eyes. 



SUMMARY 



In an earlier experiment normal dogs showed a surprising lack 

 of superiority over blind dogs in learning to make complicated 

 adjustments. A normal dog gave evidence that he did not 

 depend on vision in making many of his ordinary responses in 

 the field. 



In another earlier experiment a 4 to 1 difference in luminous 

 intensity, presented in addition to the difference in outline, en- 

 abled a dog to discriminate between a circle and its equivalent 

 square. When the stimuli were equated in luminous intensity, 

 discrimination ceased, and was not reestablished in 600 trials. 

 The factor of refractive aberration was not determined. 



In the present work, two chickens and one monkey learned 

 in 300 to 400 trials to distinguish a striate field from a plain field 

 equal to the former in area, outline, color and mean brightness. 

 A dog with two emmetropic eyes failed to learn the problem in 

 over 1,000 trials, although the striae on the positive field were 

 made nearly six times as large for him as for the other animals. 



