354 AN AMERICAN TEXT- BOOK OF PHYSIOLOGY. 



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

 of Jhe horizontal line in the same way as the outside lines in Figure 168 

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



Fig. 171— Illusion of space-perception. 



Eintlioven 1 has recently explained this phenomenon as dependent upon 

 indistinct vision in the lateral portions of the retina which causes the blurred 

 images of the ends of the line a to appear nearer together than those of the 



line h. This effect of indistinctness 

 of outline can be illustrated by photo- 

 graphing the lines more or less out 

 of focus as shown in Figure 172 a. A 

 similar explanation is given by Ein- 

 thoven for the illusion of subdivided 

 Space described on p. 351. 



Perception of Distance. — The 

 retinal image gives us, as Ave have 

 seen, no direct information as to the 

 distance of the object from the eye. 

 This knowledge is, however, quite as 

 important as that of position in a plane 

 perpendicular to the line of vision, and 

 we must now consider in what way it 

 is obtained. The first fact to be noticed 

 is that there is a close connection be- 

 tween 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 niie at a 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 thi' size and distance of objects shows that the knowledge of this rela- 

 tion 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 log, are 

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



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



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



1 Pfliiger's Archir, lxxi. S. 1. 



Pig. 172.— Illustrating Einthoven'a explanation of 

 space illusions through indistinct vision. 



