SECTION X 

 VISUAL JUDGMENTS 



LOCALISATION AND PROJECTION. Much discussion has been wasted on 

 the question why we see things upright while the images on our retina are 

 inverted. The answer is a simple one. We do not look at nor are we con- 

 scious of the image on our retina. When we say that we see anything we 

 are not expressing merely a sensation, but we are giving an interpretation 

 of certain sensations in the light of long experience which has involved a large 

 number of sensations besides that of vision. Thus a new-born child sees, 

 i.e. receives images on its retina which excite impulses in the brain, but it is 

 unable to interpret anything that it sees. In the first few months there is 

 indeed no connection between the visual sensations and eye movements ; 

 it is only about the third month after birth that the child will follow a lighted 

 candle or bright object with its eyes, and this association of ocular movements 

 with retinal impressions gradually extends also to many other movements. 

 The continual and at first apparently aimless movements of the infant bring 

 in a flood of muscular and tactile impressions which only after many trials 

 are recognised as corresponding with sensations arriving from the eyes. It 

 at first finds that with the right hand it can touch objects lying on the right 

 side of the field of vision. It becomes conscious therefore, not of the left side 

 of its retina, but of a series of objects which have distinct relations to its right 

 hand, and of a 1 certain thing seen outside itself. The projection and localisa- 

 tion of visual impressions are therefore not intuitive or innate qualities 

 attached to stimulation of each point of the retina, but are the result of 

 experience, the testing and comparing of visual sensations with tactile and 

 muscular sensations from all parts of the body. From these experiences 

 we learn to associate stimulation, say, of the right side of the retina with 

 the presence of objects lying in front of and to the left of the body, and to 

 project our visual sensations in this direction. If, for instance, we press the 

 finger, with closed eyelids, on the outer side of the right eyeball, a luminous 

 ring, or phosphene, will be seen apparently towards the left, i.e. the region 

 whence the pressed- upon part of the retina will be normally stimulated by 

 rays of light. 



The projection of visual impressions is well shown in Scheiner's experiment. Two 

 needles are placed one behind the other on a wooden rod, one at 18 cm. and the other 

 at a distance of 60 cm. from the eye. One eye is closed, and then a card is held before 

 the other. In the card two small holes are pierced by means of a needle at a distance 



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