812 THE EYE AND VISION [CH. LVI. 



The importance of binocular vision is very great. If an object is 

 looked at with one eye only, it is impossible to estimate its distance 

 by the sense of vision alone. For instance, if one eye is closed 

 and the other looks at a wire or bar, it is impossible to tell 

 whether, if some one drops a small object, it falls in front of or 

 behind the bar. 



Visual judgments are not always correct; there are a large 

 number of puzzles and toys which depend on visual illusions. One 

 or two of the best known are represented in the accompanying 

 diagrams. 



FIG. 603. Parallel puzzle. 



In fig. 602, A, B, and c are of the same size ; but A looks taller 

 than B, while c appears to cover a less area than either. The sub- 

 division of a space or line increases its apparent size or length. 

 In fig. 602 D, ah is equal to be. Vertical distances also are usually 

 over-estimated. 



In fig. 603, the horizontal lines are parallel, though they do not 

 appear so, owing to the mind being distracted by the intercrossing 

 lines. 



