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VISION 



The two eyes are very closely associated in their movements. Under normal 

 circumstances the line of vision of the one cannot be directed to a point 

 higher than that to which the other is directed at the same time, and the two 

 cannot be made to diverge. 



Theoretically, by appropriately combined action of its six muscles, each eye 

 can be turned in any direction and rotated on any axis; but the actual 



movements are few in comparison 

 with those theoretically possible. In 

 general we may say that only move- 

 ments with the lines of vision paral- 

 lel or symmetrically converged i. e., 

 directed toward a point in the mid 

 line are possible. Convergence of 

 the lines of vision toward a point not 

 in the mid line is always associated 

 with great effort, and as a rule i 

 obviated by moving the head and 

 thus avoiding, as we are always in- 

 clined to do, extreme movements of 

 the eyes. 



This limitation of the eye move- 

 ments is of very great importance for 

 visual perceptions; for the connection 



vision, after Bering. between the retinal images and the po- 



sition of the eyes is thereby rendered 



more constant than would be the case if all theoretically possible movements 

 were carried out. 



2. SIGNIFICANCE OF EYE MOVEMENTS FOR THE OUTWARD 

 PROJECTION OF VISUAL PERCEPTIONS 



It is evident from the optical principles of the eye that the images 

 thrown on the retina by refraction of light are always reversed, and yet we 

 always see the objects to which the images correspond right side up. The 

 explanation of this phenomenon has been much discussed, and yet is all very 

 simple. 



The newborn child sees, but understands nothing of what it sees. In- 

 cluded in the knowledge which the child gains by experience with the sense 

 of sight is the knowledge of the position of things. But this knowledge the 

 child does not obtain by the sense of sight alone; the bodily movements play 

 a determining part as well. When the child looks at its nurse the image is 

 upside down on the retina. But if it should wish to touch the nurse's head 

 with its hand, it must move its arm in the right direction. In this way a 

 definite connection is established between the retinal image and the move- 

 ments, and the child learns to project its visual impressions outward in the 

 proper direction. 



The reason then why we see all objects right side up is, that in developing 

 our ability to recognize external objects and their position in space, we have 



