BINOCULAK VISION. 713 



and presents the errors of appreciation which attend monocular vision, such 

 as a want of exact estimation of the solidity and distance of objects. It is 

 stated as the rule that when strabismus of long standing is remedied, as far 

 as the axes of the eyes are concerned, by an operation, binocular vision is not 

 restored. This is explained upon the supposition that the perceptive power 

 of the retina of the affected eye has been gradually and irrecoverably lost 

 from disuse. In normal binocular vision the images are formed upon the 

 f ovea centralis of each eye ; that is, upon corresponding points, which are, 

 for each eye, the centres of distinct vision. The concurrence of both eyes is 

 necessary to the exact appreciation of distance and form ; and when the two 

 images are formed upon corresponding points, the visual centre receives a 

 correct impression of a single object. When vision is perfect, the sensation 

 of the situation of any single object is referred to one and the same point ; 

 and the impression of a double image can not be received unless the condi- 

 tions of vision be abnormal. 



Corresponding Points. While it is evident, after the statements just 

 made, that an image must be formed upon the fovea of each eye in order to 

 produce the effect of a single object, it becomes important to ascertain how 

 far it is necessary that the correspondence of points be carried out in the 

 retina. It is almost certain that for absolutely perfect, single vision with the 

 two eyes, the impressions must be made upon exactly corresponding points, 

 even to the ultimate, sensitive elements of the retina. It may be assumed, 

 indeed, that each rod and each cone of one eye has its corresponding rod and 

 cone in the other, situated at exactly the same distance and in correspond- 

 ing directions from the visual axis. When the two images of an object are 

 formed upon these corresponding points, they appear as one ; but when the 

 images do not correspond, the impression is as though the images were 

 formed upon different points in one retina, and of necessity they appear 

 double. 



The effect of a slight deviation from the corresponding points may be 

 illustrated by the following experiment : If a small object, like a lead-pencil, 

 held at a distance of a few inches, be fixed with the eyes, it is seen distinctly 

 as a single object. Holding another small object in the same line, a few 

 inches farther removed, when the first is seen distinctly, the second appears 

 double. If the second object be fixed with the eyes, the first appears double. 

 It is evident here, that when the axes of the eyes bear upon one of these ob- 

 jects, the images of the other must be formed at a certain distance from the 

 corresponding retinal points. 



The Horopter. The above-mentioned experiment affords an explanation 

 of the horopter. If both eyes be fixed upon a point directly in front and be 

 kept in this position, an object moved to one side or the other, within a cer- 

 tain area, may be seen without any change in the direction of the axis of vis- 

 ion ; but the distance from the eye at which there is single vision of this 

 object is fixed, and at any other distance the object appears double. The 

 explanation of this is that at a certain distance from the eye, the images are 

 formed upon corresponding points in the retina ; but at a shorter or longer 



