Photoreception 40 j 



visual field is almost entirely binocular. Hares have monocular fields of 190 

 degrees which oxerlap to form small binocular fields both anteriorly and 

 posteriorly. 



Eye Movements and the Fovea. Movement of the eyes may be involuntary 

 or voluntary. Involuntary movements are those which form automatic reflexes 

 for the purpose of maintaining the visual field constant during locomotion 

 and during passive jogglings of the head and body. An excellent example of 

 such mo\'ement is that which we make when we look another person in the 

 eye and shake our head in the gesture of "no." This eye movement is auto- 

 matic and almost uncontrollable. Moving pictures taken from a moving, jog- 

 gling vehicle do not haxe this automatic adjustment, and the absence is 

 both noticeable and unpleasant to the observer. 



During \'oluntar\' moxements the two eves may mo\e independently with 

 no coordination or may be coordinated to various degrees. In most lizards 

 and in birds the movement is independent and uncoordinated; in some fishes 

 and in chameleons it is independent but coordinated when objects are 

 observed in the binocular field; and in man it is never independent but is 

 always coordinated in such a manner that the same point of an object is al- 

 ways focused simultaneously on the two foveas. 



In birds, moving objects are usually followed by movement of the entire 

 head, and fixation ma\' be either monocular or binocular. Many birds (ac- 

 cipitrines, swallows, etc., mostly flight feeders) have two foveas in each eye, 

 one of which is used monocularly and one binocularly. Since the eyes are 

 directed at an angle of as much as 45 degrees or more from each other, the 

 usual central foveas are useful only monocularly, but the second foveas are 

 placed on the temporal surfaces of the retinae so that they may be used si- 

 multaneously in a binocular manner (Fig. 111). This arrangement is the 

 so-called "visual trident" by means of which a bird may have three objects 

 focused simultaneously on its four foveas, one object on two foveas, and one 

 each on the other two. 



Depth and Solidity Perception. Perception of depth and distance is ob- 

 tained either binocularly or monocularly. Most of the clues to depth are em- 

 ployed in both monocular and binocular vision and may be detected in two- 

 dimensional photographs or in good paintings. Perception depends on the fol- 

 lowing: (1) Retinal image size. The larger the image the closer the object. 

 (2) Perspective. Horizontal lines tend to meet at the horizon, and the amount 

 of taper is an index of size and distance. (3) Overlap and shadow. Objects 

 whose images or shadows overlap other objects are nearer than those objects. 

 Distances with few overlaps, as over water with no interxening objects, 

 therefore tend to be underestimated. (4) Vertical nearness to the horizon. 

 Our line of sight tilts toward the ground for near objects but is raised for dis- 

 tant objects. (5) Aerial perspective. Objects with dim or bluish outlines seem 

 more distant because long atmospheric pathways make them appear so. In 

 very clear air, distances tend to be underestimated; in fog or mist, overesti- 

 mated. (6) Parallax, the most important of all monocular clues. This is the 

 change in the apparent angle, at the eye, between near and far objects, pro- 

 duced by lateral movement of the observer's head or body. This is the princi- 

 pal clue which permits one-eved men to mo\'e about crowded rooms as well 

 as they do. 



