254 



THE HUMAN MECHANISM 



sensations from the two eyes are produced only by solid 

 objects. Again, when we look at any point on the near edge 

 of a box the two eyes are converged by their muscles to a 

 greater extent than when we look at a point on the far edge, 

 and we have learned that these different pulls of muscles and 

 positions of eyeballs indicate that the object is not flat, but 

 has depth. The importance of binocular vision in the estima- 

 tion of depth or distance from the eye is most strikingly 

 illustrated by attempting, with one eye closed, to bring 

 together the points of two pencils held in the hands and 

 moved from side to side at arm's length. 



Consequently when we look at anything we get a number 

 of sensations ; from the retina, those of color and the posi- 

 tion of the color spots with 

 reference to one another ; from 

 the muscular efforts of accom- 

 modation and of convergence 

 of the eyeballs, those which 

 reveal the property of depth 

 in what we see. And from 

 all of these, fused together 

 and interpreted in the light 

 of experience, we construct a 



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FIG. 99 



visual judgment of the nature 



of the object. 



16. Optical illusions. That 



our vision is essentially the 



result of unconscious judgments is strikingly shown by the 

 fact that these sometimes deceive us. Thus the parallel 

 vertical lines in Fig. 99, when crossed by the oblique lines, seem 

 to be inclined toward each other. The retinal images of the 

 lines are parallel, and we falsely judge them inclined, this 

 error of judgment arising from the presence of the oblique 

 lines. In other words, our final idea of the lines does not 

 correspond to their image on the retina. 



