CHAP, in.] SIGHT. 139 



lower half to the lower half, the right side to the right side, 

 and the left side to the left side. But when we turn to the 

 structure of the retina we find that the left or nasal side of the 

 right eye, since it contains the entrance of the optic nerve, is com- 

 parable with, not the left, but the right or nasal side of the left 

 eye, and in like manner the right or temporal side of the right 

 eye is comparable with the left or temporal side of the left eye. 

 Hence, considered in relation to the structure of the retina, the 

 corresponding points appear to be reversed from side to side, 

 though not from top to bottom. While the upper half of the 

 retina of the left eye corresponds to the upper half of the retina 

 of the right eye, and the lower to the lower, the nasal side of the 

 left eye corresponds with the temporal side of the right, and the 

 temporal of the left with the nasal side of the right. 



It will be observed that in each eye a vertical plane through 

 the visual axis (v. I. in Fig. 154) cuts the retina in a vertical line 

 v. m., which divides the retina into two lateral, temporal and nasal, 

 halves, each temporal and each nasal half corresponding with the 

 nasal and temporal half respectively of the other eye. When the 

 visual axes of the two eyes are parallel, the two vertical planes in 

 question are parallel to the median plane and to each other. 

 Further, a horizontal plane drawn through the visual axis at right 

 angles to the above vertical plane cuts the retina in a horizontal 

 line li. m. ; and this also divides the retina into two halves, an 

 upper and lower half, the upper and the lower halves of both 

 retina being corresponding. These two lines, each of which may 

 be considered as a series of corresponding points, are sometimes 

 spoken of as lines of separation. 



The blending of the two sensations into one occurs, we repeat, 

 only when tha two images of an object fall on corresponding 

 points of the two retinas. Hence it is obvious that in single 

 vision with two eyes the ordinary movements of the eyeballs must 

 be such as to bring the visual axes to converge at the object 

 looked at so that the two images may fall on corresponding points. 

 When the visual axes do not so converge, and when therefore the 

 images do not fall on corresponding points, the two sensations are 

 not blended into one perception, and vision becomes double. It is 

 therefore important to study in some detail the movements of the 

 eyeballs, by means of which, in ordinary vision, the relative posi- 

 tions of the two retinas are so carefully adjusted that we habitually 

 see objects single not double. 



788. The movements of the Eyeball. As we have said, the 

 movements of the eyeball are movements of rotation round an 

 immobile centre, the centre of rotation ; but these movements are 

 limited in a particular way, and it is necessary to pay attention to 

 their characters and limitation. 



One position of the eyeball, for reasons which we shall see 

 presently, is called the primary position, and it will be desirable 



