554 VISION 



If, after looking for a moment at the sun until the eye is fatigued, the 

 eyes be turned toward a uniformly lighted wall, one sees there an after-image 

 of the sun, the size of which depends upon the distance of the wall the farther 



away the wall the larger the after-image 

 oooooo o (H. Meyer). 



We shall return again to the condi- 



FIG. 244. After Hering. tions for the perception of depth and the 



laws by which we judge the apparent dis- 

 tance of an object. From the facts just presented, which are exactly the 

 same with and without accommodation, it follows that a retinal image of 

 a given size is projected in different sizes according to the position of the 

 visual lines, the object appearing smaller when they are converged, larger 

 when they are parallel. 



The size of the retinal image therefore is not always the determining 

 factor in judging the size of objects. The apparent size of a well-known 

 object e. g., that of an adult man does not vary noticeably when it is seen 

 at different distances, although the size of the retinal image changes consid- 

 erably. Such peculiarities are the result 



of a gradually acquired experience. A A & 



child relying on the size of its retinal im- 

 ages misjudges the size of objects much 

 more than an adult. 



In forming judgments of linear, verti- . 



cal and horizontal distances, equally re- FIG. 245. After Heimhoitz. 



moved from us, the movements of our eyes 



play a determining part (Wundt). If we compare a space divided by points 

 or lines into intervals with an equal space not so divided, the former appears 

 greater than the latter (Hering, Fig. 244). Two squares of the same size, 

 one ruled horizontally, the other vertically, appear to be different in both 

 breadth and height (Fig. 245). In both these examples the retinal images 

 of the two objects compared are exactly equal in size, and the accommodation 

 is the same. The basis of the phenomenon appears to be that it requires less 

 muscular effort to cast the eyes over an empty space than over one interrupted 

 at certain intervals. It is as if the eye had to make a 

 fresh effort at each point in Fig. 244, and at each line 

 in Fig. 245. 



The vertical line in Fig. 246 appears longer than 

 the horizontal one because it requires greater muscular 

 effort to move the line of vision up and down than to 

 FIG. 246. move it out and in. For in the movement of the visual 



line directly upward it is necessary that two muscles 



cooperate (cf. page 550). One of these, the superior rectus, tends to turn 

 the eye upward and inward; the other, the inferior oblique, tends to turn it 

 upward and outward. Hence part of the muscular force developed in each 

 muscle is used in antagonizing the other. But in rotation of the eye directly 

 outward and inward no such compensation is necessary; hence not so much 

 muscular effort is required. The horizontal line therefore seems shorter. 



