782 



SIGHT. 



ments of the eyeballs must be such as to bring the visual axes to converge 

 at the object 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. 



FIG. 179. 



Diagram illustrating Corresponding or Identical Points: L, the left, R, the right eye ; n, nodal 

 point ; o, optic nerve (blind spot) ; x, fovea. x' y' z' in the right eye are corresponding points to x y 

 z in the left eye. v I, visual axis. The two figures below are projections of L the left and R the 

 right retina. /, fovea ; o, blind spot, a and c on the temporal side of L correspond to a' and c' on the 

 nasal side of R. vmhm, lines separating quadrants. 



Movements of the Eyeballs. 



681. The eye is virtually a ball placed in a socket, the bulb and the 

 orbit forming a ball-and-socket joint. In its socket-joint the optic ball is 

 capable of a variety of movements, but it cannot by any voluntary effort be 

 moved out of its socket. It is stated that by a very forcible opening of the 

 eyelids the eyeball may be slightly protruded ; but this trifling locomotion 

 may be neglected. By disease, however, the position of the eyeball in the 

 socket may be materially changed. 



Each eyeball is capable of rotating round an immobile centre of rotation, 

 which has been found to be placed a little (1.77 mm.) behind the centre of 

 the eye ; but the movements of the eye round the centre are limited in a pecu- 



