314 ADAPTATIONS TO SPACE AND MOTION 



distance. As an object approaches, it appears to grow. A closely related 

 cue is: 



B. Perspective. All horizontal lines, if produced, appear to meet at the 

 horizon. An object appears farther away if its horizontal contours are 

 close to meeting. We know which end of an object is nearer to us, from 

 the direction in which the object appears to taper. 



C. Overlap and Shadow. If one object hides part of a second object, 

 it must be the nearer of the two. So also if it casts a shadow on the sec- 

 ond object. The more overlaps there are in a visual field, the greater 

 seems to be the distance to the farthest object. This is why distances over 

 water, with no intervening objects, tend to be underestimated. 



D. Vertical Nearness to the Horizon. When we are looking at nearby 

 objects, our line of sight tilts toward the ground. More distant objects 

 are seen at apparently higher levels, for in looking at them the line of 

 sight must be elevated. 



E. Aerial Perspective. Objects appear farther away if their outlines are 

 hazy and their surfaces dim or bluish, for long atmospheric pathways 

 create such appearances. Colors are affected by aerial perspective and 

 become unsaturated at a distance. Distances through exceptionally clear 

 air tend to be underestimated; distances through mist, overestimated. 



F. Parallax. This, the most important of all monocular factors, is the 

 change in the apparent angle, at the eye, between a near and a far object, 

 produced by a lateral movement of the observer's body or head. As we 

 move our heads from side to side, near objects seem to move extensively 

 in the opposite direction as compared with far objects, while the latter 

 seem to move slightly, in the same sense as the head movements, in rela- 

 tion to the nearer objects. It is chiefly this cue which enables a one-eyed 

 man to move about in an unfamiliar roomful of furniture without bump- 

 ing into things any more often than a two-eyed person. 



The one-eyed person, however, may have considerable difficulty with 

 the common parlor trick in which one attempts to bring two pencil-points 

 together with the arms outstretched. Binocular perception of distance — 

 shcJrt distance, at any rate — is infinitely finer than monocular. What is 

 its special basis? 



In binocular vision, whenever the eyes accommodate for a particular 

 distance, they also converge to a degree that aims the two foveal lines of 

 sight at a common point at that distance. In looking from one object to 

 another which is at a different distance, the extent of convergence either 



