BINOCULAR VISION. 119 



points, the brain receives a correct impression of a single 

 object. "When our vision is perfectly normal, the sensation 

 of the situation of any single object is referred to one and 

 the same point, and we cannot receive the impression of a 

 double image, unless the conditions of vision be abnormal. 



Corresponding Points. While it requires no argument, 

 after the statements we have just made, to show 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. This leads to consid- 

 erations of very great interest and importance. It is almost 

 certain that, for absolutely perfect, single vision with the two 

 eyes, the impressions must be made upon exactly correspond- 

 ing points, even to the ultimate sensitive elements of the ret- 

 ina. We may suppose, 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 in corresponding direc- 

 tions from the visual axis. 1 When the two images of an ob- 

 ject are formed upon these corresponding points, they appear 

 as one ; but, when the images do not correspond, the impres- 

 sion 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 : We 

 fix a small object, like a lead-pencil, held at a distance of a 

 few inches, with the eyes, and see it distinctly as a single 

 object ; we hold in the same line, a few inches farther re- 

 moved, another small object ; when the first is seen dis- 

 tinctly, the second appears double ; we fix the second with 



1 It is interesting to note, in this connection, the point, shown by cases of 

 hemiopsia, that the fibres from the optic tract upon one side are probably con- 

 nected with the outer vertical half of the retina of the same side, and with the 

 inner vertical half of the retina of the opposite side. This shows that the outer 

 half of one retina corresponds to the inner half of the other. (See page 41.) 



