THE EYK AND THK PHYSIOLOGY OF VISION. 



587 



occur. For this reason ten miles on a level prairie with a 

 clear view seem much shorter than when the intervening 



Fig. 185. OPTICAL ILLUSION. THE PARALLELISM OF THE LONG LINES is DESTROYED BY 



THE CROSS-LINES. 



space is filled with a multitude of striking objects. So in 

 Figure 186 the distance from a to b seems longer than the 

 distance from b to c. For a similar reason the first square 

 in Figure 187 seems higher and narrower than the second. 

 In Figure 188 the distance from a to b seems greater than 

 the distance from c to d. 



ViZ> 186. Fig. 187. 



Fififs. 186 and 187-7-OPTiCAL ILLUSIONS. 



4. Size. Primarily, the size of an object depends upon 

 the size of its image in the eye, but we have learned by ex- 

 perience that the size of this image will depend upon the 

 distance of the object. Therefore, when we know the dis- 

 tance we infer the size. It is the opposite, in short, of the 

 inference of distance. When we are ignorant of the dis- 

 tance we are unable to judge size closely, our inferences 

 being then based wholly upon the clearness with which it 



