138 



CORRESPONDING POINTS. 



[BOOK in. 



axes of the two eyes converge at the object, the eyeballs moving 

 accordingly. The corresponding points of the two retinas are 

 those on which the two images of the object fall when the 

 visual axes converge at the object. Thus in Fig. 154 if vl from 

 X to x and X to x' be the two visual axes, x, x' being the centres 

 of the foveae centrales of the two eyes, then, the object XYZ 

 being seen single, the point y on the one retina will ' corre- 

 spond ' to or be ' identical ' with the point y 1 on the other, and 

 the point 2 in the one to the point 2' in the other. 



:v 



m h 



m m 



FIG. 154. DIAGRAM ILLUSTRATING CORRESPONDING POINTS. 



L the left, R the right eye, n. nodal point, o. optic nerve, x. fovea. ar'yV are points 

 in the right eye corresponding to the points xyz in the left eye. v. I. visual 

 axis. The two figures below are projections of L the left and R the right 

 retina, f. fovea, o. blind spot. It will be seen'that a and c on the temporal side 

 of L corresponds to a and c' on the nasal side of R. v. m. h. m. lines of 

 separation. 



When the whole area of the retina in each eye which we use 

 for binocular vision is explored in this way we find, as follows 

 geometrically from the paths of the rays of light, that the upper 

 half of one retina corresponds to the upper half of the other, the 



