MOVEMENTS OF THE EYEBALLS 657 



It might be thought that single vision with the two eyes was due 

 to the fact that the nerve fibres from corresponding points in the two 

 retinae pass to and terminate in identical nervous structures in the 

 brain. This is not the case. Single vision is largely the result of 

 experience, and we can show by experiment that both elements are 

 present in the fused sensation obtained from the two eyes. Thus if 

 the left eye is directed on to a red surface and the right eye on to a 

 blue surface, we may obtain either a fusion of the fields with the 

 production of a purple colour, or a struggle of the two fields so that 

 the whole field appears sometimes red and sometimes blue. In the 

 same way if two figures, lying side by side, one ruled with vertical lines 

 and the other with horizontal lines, be looked at so that the image of 

 the right figure falls on the right retina and of the left figure on the 

 left retina, we do not see a single figure with the lines crossing one 

 another so as to form a network, but we again obtain a struggle so 

 that first one figure is seen with the vertical lines prominent, and then 

 this wanes and is succeeded by the other figure in which the horizontal 

 lines are prominent. Which figure we see most easily seems to be 

 dependent on the direction of the eye movement. If the eyes are 

 moved vertically upwards and downwards the figure with the vertical 

 lines comes more into prominence, while the figure with the horizontal 

 lines is seen when the eyes are moved from side to side. 



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