SEC. 9. BINOCULAR VISION. 



587. So far we have treated of vision as if it were carried 

 out by means of one eye and have only incidentally referred to 

 our possessing two eyes. Our ordinary vision is, however, carried 

 out by means of two eyes, our vision is binocular not monocular; 

 and to the characters of this binocular vision we must now turn. 

 In dealing with monocular vision we rarely have occasion to 

 refer specially to the movements of the eyeball ; but in binocular 

 vision these play an important part ; and even before we go into 

 details, it will be desirable to point out not only certain general 

 facts, but also the meaning of certain terms which we shall have 

 to use. 



The eye is virtually a ball placed in a socket, the bulb or 

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

 socket joint the eyeball is capable of various movements, but 

 these are limited to those of rotation within the socket; the 

 eyeball cannot by any voluntary effort be moved out of its 

 socket. It is stated that by a very forcible opening of the eye- 

 lids 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. 



The movements of rotation to which the eyeball is thus 

 limited are carried out round a centre in the eye which is termed 

 the centre of rotation, and which has been determined to lie in 

 the vitreous humour about 13.5 mm. behind the anterior surface 

 of the cornea, not quite 2 mm. behind what, though the eyeball 

 is not a sphere, may be considered as the geometric centre of the 

 eyeball ; it is of course quite different from the optical centre 

 or nodal point of the diagrammatic eye ( 527). 



When we, in looking, direct our vision to a point, a line 

 drawn from such a point, which we may call the fixed point of 

 vision, to the centre of rotation, is called the visual axis ; pro- 

 longed past the centre of rotation it meets the retina in the 

 centre of the fovea centralis ; hence in the view of those who 

 hold that the optic axis, the line on which the dioptric surfaces 

 of the eye are centred, meets the retina on one side of the 

 fovea, the visual axis does not coincide with, and is different 



939 



