SINGLE VISION. 665 



again. The cause of this phenomenon is simply that the 

 portion of retina which is occupied by the entrance of the 

 optic nerve, is quite blind ; and therefore that when it 

 alone occupies the field of vision, objects cease to be 

 visible. 



Of the Simultaneous Action of the two Eyes. 



Although the sense of sight is exercised by two organs, 

 yet the impression of an object conveyed to the mind is 

 single. Yarious theories have been advanced to account 

 for this phenomenon. By Grail, it was supposed that we 

 do not really employ both eyes simultaneously in vision, 

 but always see with only one at a time. This especial 

 employment of one eye in vision certainly occurs in 

 persons whose eyes are of very unequal focal distance, but 

 in the majority of individuals both eyes are simultaneously 

 in action in the perception of the same object ; this is 

 shown by the double images seen under certain conditions. 

 If two fingers be held up before the eyes, one in front of 

 the other, and vision be directed to the more distant, so 

 that it is seen singly, the nearer will appear double ; while, 

 if the nearer one be regarded, the most distant will be 

 seen double; and one of the double images in each case 

 will be found to belong to one eye, the other to the other 

 eye. 



Single vision results only when certain parts of the two 

 retinae are affected simultaneously ; if different parts of 

 the retinae receive the image of the object, it is seen 

 double. The parts of the retinae in the two eyes which 

 thus correspond to each other in the property of referring 

 the images which affect them simultaneously to the same 

 spot in the field of vision are, in man, just those parts 

 which would correspond to each other if one retina were 

 placed exactly in front of, and over the other (as in 

 fig. 1 8 1, c). Thus, the outer lateral portion of one eye 

 corresponds to, or, to use a better term, is identical with, 



