154 THE HOEOPTER. [BOOK in. 



given position of the eyes, are projected on to corresponding points 

 of the retina ; hence its determination in any particular case is 

 simply a matter of geometrical calculation. In some instances it 

 becomes a very complicated figure. The case whose features are 

 most easily grasped is that of a circle drawn in the plane of the two 

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

 the nodal points of the two eyes such as is shewn in. Fig. 157. It 

 is obvious from geometrical relations that the two images of any 

 point in such circle, the rays from which can enter the two pupils 

 and fall on the two retinas, will fall on corresponding points of the 

 two retinas. When we study the various horopters of the 

 several positions which the two eyes can take up, we find that the 

 characters of the horopter are adapted to the needs of our daily 

 life. Thus in the position assumed by the two eyes when we 

 stand upright and look at the distant horizon the horopter is 

 (approximately, for normal emme tropic eyes) a plane drawn 

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

 the advantage of this is obvious. 



Nevertheless, in most positions of the eyes a large number of 

 the images which make up the binocular visual field, do not lie 

 on any horopter, do not fall on corresponding points, and give 

 rise not to one sensation only but to two sensations differing to 

 a certain extent from each other. A great deal of what we see 

 is seen double by us, we receive from many objects two unequal 

 impressions ; but the inequality chiefly serves to give an ap- 

 pearance of " solidity " to the objects, to assist in our judgment 

 of solidity. To the consideration of these and other visual judg- 

 ments as well as of some other psychological features of vision 

 we must now turn. 



