548 BINOCULAR VISION. [BOOK m. 



becomes a very complicated figure. The case whose features are 

 most easily grasped is a cii^le drawn in the plane of the two 

 visual axes through the point of the convergence of the axes and 

 the optic centres of the two eyes. It is obvious from geometrical 

 relations that in Fig. 76 the images of any point in the circle will 

 fall on corresponding points of the two retinas. When we stand 

 upright and look at the distant horizon the horopter is (approxi- 

 mately, for normal emmetropic eyes) a plane drawn through 

 our feet, that is to say, is the ground on which we stand; the 

 advantage of this is obvious. 



