784 



THE EYE AND VISION. 



[CH. LV. 



another (as in looking at a small object near to and immediately 

 on the same level as the eyes). 



Tertiary positions are those in which the visual lines are not 

 horizontal, and converge towards one another (as in looking at 

 the tip of the nose). 



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



Fig. 594. Identical points of the retinae. 



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 



disregarded by the brain, which 

 only pays attention to those 

 images which fall on correspond- 

 ing points. 



The following diagrams will 

 assist us in understanding more 

 fully what is meant by corre- 

 sponding or identical points of 

 the two retinae. 



If R and L (fig. 594) repre- 

 sent the right and left retinae respectively, and 0' the two 

 yellow spots are identical so are A 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' are not identical. 

 Fig. 595 shows the same thing in rather a different way ; 

 A and B represent a horizontal section through the two retinae ; 

 the points a a', b b', and c c', being identical. In the lower part 



j. 595. Diagram to show the correspond- 

 ing parts of both retinae. 



