SINGLE VISION. 521 



example, we close the left eye, and direct the axis of the right 

 eye steadily towards the circular spot above represented, while 

 the page is held at a distance of about six inches from the 

 eye, both dot and cross are visible. On gradually increasing 

 the distance between the eye and the object, by removing the 

 book farther and farther from the face, and still keeping the 

 right eye steadily on the dot, it will be found that suddenly 

 the cross disappears from view, while on removing the book 

 still farther, it suddenly comes in sight 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. 

 Various theories have been advanced to account for this phe- 

 nomenon. By Gall, 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 un- 

 equal 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. 188, c). Thus, the outer lateral portion of 

 one eye corresponds to, or, to use a better term, is identical 

 with the inner portion of the other eye ; or a of the eye A 

 (Fig. 188) with a' of the eye B. The upper part of one retina 

 is also identical with the upper part of the other ; and the lower 

 parts of the two eyes are identical with each other. 



