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PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



coincide perfectly ; and this takes place when the retinal 

 images fall on corresponding points of the two retinae. 

 It is necessary to define these exactly. 



Corresponding Points. — Corresponding points are 

 points exactly similarly situated in the two retinas. The 

 foveas are, of course, par excellence corresponding points, 

 and all other corresponding points are symmetrically 

 arranged about these. If R and L (Fig. 90) represent 

 projections of the two retinas, and c c the centers of the 

 foveas, and vertical and horizontal lines be drawn 

 through the central spots, then points similarly situated 

 in reference to these — viz., e e', d d'- — are corresponding 

 points. Or, suppose the two retinas be placed one on 



Fig. go. — Diagram showing corresponding halves of the retinas. 



the other in geometric coincidence, then the points — 

 the rods and cones — which coincide are corresponding 

 rods or cones. It follows that the two right or shaded 

 halves are corresponding halves, and similarly the two 

 left or unshaded halves — i. e., points similarly situated 

 in the two right halves or left halves — are correspond- 

 ing. But the two inner or nasal halves have no corre- 

 sponding points, nor have the two external or temporal 

 halves any correspondents. 



The Third Law of Vision ; the Law of Corre- 

 sponding Points. — We restate now the conditions of 

 single vision as a law. When the two retinal images of 



