Chap, hi.] 



SIGHT. 



943 



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

 axes converge at the object. Thus in Fig. 158 if vl from X to 

 x and Xto x' be the two visual axes, x, x' being the centres of 

 the foveas centrales of the two eyes, then, the object XYZ being 

 seen single, the point y on the one retina will ' correspond ' to 

 or be ' identical ' with the point y' on the other, and the point z 

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



m m 



Fig. 158. Diagram illustrating Corresponding Points. 



L the left, B the right eye, n. nodal point, o. optic nerve, x. fovea, x'y'z' are 

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

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

 retina. /. 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 B. v. m. h. m. lines of sepa- 

 ration. 



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 lower half to the lower half, the right side to the 



