588 KARL T. WATJGH 



what less favorable to binocular vision than in the mouse. The 

 snout of the mouse is proportionately longer than that of the 

 squirrel, but the bridge between the eyes is so much lower that 

 there is a large field of vision for two eyes above the head, which 

 the squirrel does not possess. 



When the squirrel under observation was approached from the 

 side, he would sit on his haunches, lift up his head and show all 

 signs of attention. When I would kneel on the ground within 

 six feet of him and make no movement, he would remain with one 

 side of his head toward me, using only one eye. When a movement 

 was made by waving the hand back and forth, he would turn his 

 head directly toward me in a position where both eyes were 

 equally visible. This is a reaction very similar to that in the mouse. 



Two explanations of this reaction suggest themselves : (1) The 

 movement is made for the purpose of getting perspective which 

 would aid in the perception of distance. Here convergence 

 would be implied in the way that it occurs in human beings. 

 (2) The head movement is made for purposes of orientation pre- 

 paratory to turning the body in the direction of the stimulus and, 

 perhaps, approaching it. The forward movements and turnings 

 of the animal are executed with reference to a median plane, to 

 which the precise relation of an object in space is more easily 

 determined when the object is seen with both eyes. For deter- 

 mining the space relation of two objects both in the median plane, 

 the one factor of location of images on the retina is adequate. 

 Obviously a crawling animal like the mouse is more concerned with 

 accuracy in its right and left movements than in movements in the 

 vertical plane. 



Of the two explanations gi^en the latter is the more likely to 

 prove correct. It is in accord with our experimental results in 

 depth perception in the mouse, and further, the idea of conver- 

 gence is not entirely compatible with a retina without a fovea, 

 homogeneous throughout and of which the habits of the animal 

 would demand only the function of communicating in a rough 

 way the general nature of the object and its direction. 



Perception of distance adequate to the animal's needs may be 

 obtained through a synthetic correlation of retinal impressions 

 and motor impulses of monocular accommodation. 



