THE SENSE OF VISION. 799 



between the extremities of the oblique lines seems also to affect our estimate 

 of the horizontal line in the same way as the outside lines in Figure 256 

 influence our judgment of the length of the line between them. 



Perception of Distance. ~The retinal image gives us, as we have seen, 

 no direct information as to the distance of the object from the eye. This 



FIG. 257. To illustrate so-called " confluxion " in space-perception (Muller-Lyer). 



knowledge is, however, quite as important as that of position in a plane per- 

 pendicular to the line of vision, and we must now consider in what way it is 

 obtained. The first fact to be noted is that there is a close connection between 

 the judgments of distance and of actual size. A retinal image of a given 

 size may be produced by a small object near the eye or by a large one at a 



FIG. 258. To illustrate the influence of angles upon the apparent length of lines (Muller-Lyer). 



distance from it. Hence when we know the actual size of any object (as, for 

 example, a human figure) we judge of its distance by the size of its image on 

 the retina. Conversely, our estimate of the actual size of an object will 

 depend upon our judgment of its distance. The fact that children constantly 

 misjudge both the size and distance of objects shows that the knowledge of 



FIG. 259. Illusion of space-perception. 



this relation is acquired only by experience. If circumstances mislead us 

 with regard to the distance of an object, we necessarily make a corresponding 

 error with regard to its size. Thus, objects seen indistinctly, as through a fog, 

 are judged to be larger, because we suppose them to be farther off, than they 

 really are. The familiar fact that the moon seems to be larger when near the 

 horizon than when near the zenith is also an illustration of this form of illu- 



