SEC. 4. BINOCULAR VISION. 



Corresponding or Identical Points. 



Though we have two eyes, and must therefore receive from 

 every object two sets of sensations, our perception of any object is 

 under ordinary circumstances a single one ; we see one object, not 

 two. By putting either eye into an unusual position, as by squint- 

 ing, we can render the perception double; we see two objects 

 where one only exists. From which it is evident that singleness of 

 perception depends on the image of the object falling on certain 

 parts of each retina at the same time, these parts being so related 

 to each .other, that the sensations from each are blended into one 

 perception ; and it is also evident that the movements of the eye- 

 balls are adapted to bring the image of the object to fall on these 

 'corresponding' or 'identical* parts, as they are called, of each 

 retina. 



When we look at an object with one eye the visual axis of that 

 eye is directed to the object, and when we use two eyes the visual 

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

 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. 74, if Cc, Cc l be the two 

 visual axes, c, c l being the centres of the foveae centrales of the two 

 eyes, then, the object AGE being seen single, the point a on the 

 one retina will ' correspond ' to or be ' identical ' with the point a v 

 on the other, and the point b in the one to the point b l in the other. 



