852 



THE EYE AND VISION 



[CH. LVIII. 



It is possible to conceive positions of the eyeballs in which the 

 visual lines diverge from one another; but such positions do not 

 occur in normal vision in man. 



Both eyes are moved simultaneously, even if one of them 

 happens to be blind. They are moved so that the object in the 

 outer world is focussed on the two yellow spots, or other corre- 

 sponding points of the two retinae. The images which do not fall 

 on corresponding points are seen double, but these are to a great 

 extent disregarded by the brain, which pays particular attention to 

 those images which fall on corresponding points. 



The accompanying diagrams will assist us in understanding what 

 is meant by corresponding or identical points of the two retinae. 



If E and L (fig. 538) represent the right and left retinse 

 respectively, and 0' the two yellow spots are identical ; so are A 



FIG. 538. Identical points of the retinse. 



and A', both being the same distance above and 0'. But the 

 corresponding point to B on the inner side of in the right retina, 

 is B', a point to the same distance on the outer side of 0' in the left 

 retina ; similarly C and C' are identical. The two blind spots X and 

 X 7 are not identical. 



Fig. 539 shows the same thing in rather a different way ; A and 

 B represent horizontal sections through the two retinas ; the points 



a a', b b', and c c', being identical. 

 In the lower part of the diagram is 

 shown the way in which the brain 

 combines the images in the two retinae, 

 one overlapping so as to coincide with 

 the other. 



The Horopter is the name given to 

 the surface in the outer world which 

 contains all the points which fall on 

 the identical points of the retinse. 



The shape of the horopter will vary with the position of the eye- 

 primary position, and in the first variety of the 



Fio. 539. Diagram to show the correspond- 

 ing parts of both retinae. 



balls. . In the 





