942 CORRESPONDING POINTS. [Book hi. 



object can fall on any part of the two retinas at the same time, 

 and in these creatures vision is wholly monocular. 



Fig. 157. The visual fields (fields of sight) of the two eyes when 



THE EYES CONVERGE TO THE SAME FIXED POINT. ( Allbert. ) 



The shaded part is that common to the two eyes. /, the fixed point, corre- 

 sponding to the fovea of each eye ; x, the blind spots of the two eyes. 



§ 589. Corresponding or Identical Points. Though when 

 we use two eyes, we must receive from every object in the field 

 of sight common to the two eyes two sets of visual impulses, 

 indeed we may say two sets of sensations, our perception of the 

 object is under ordinary circumstances a single one ; we see 

 one object, not two. By putting either eye into an unusual 

 position, as by squinting, we can render the perception double ; 

 we see two objects where one only exists. This singleness of the 

 sensation under ordinary circumstances shews that certain parts 

 of each retina are so related to each other that when an image of 

 an object falls on these parts at the same time, the two sets of sen- 

 sations excited in the two parts are blended into one ; such parts 

 are spoken of as corresponding parts ; they have also been called 

 identical parts. Since in the ordinary movements of the eyes 

 we see objects single, and do not receive double impressions 

 unless we move the eyes in an unusual manner, it is obvious 

 that the movements of the eyeballs and these corresponding- 

 parts of the two retinas are so related, the one to the other, that 

 the former bring the images of objects to fall on the latter. 



We can easily determine which are the corresponding parts 

 of the two retinas by tracing out the paths of the rays of light 

 falling on the two retinas, § 528. As we have said, 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 moving 

 accordingly. The corresponding points of the two retinas are 



