CHAP. ui.J 



SIGHT. 



137 



between the eyes ; in some of the lower animals the position of 

 the eyes is so completely lateral that no rays of light proceeding 



FIG. 153. 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, f, the fixed point, corre- 

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



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

 same time, and in these creatures vision is wholly monocular. 



787. 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 per- 

 ception 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 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 sensations 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 move- 

 ments 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, 706. 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 



