806 



THE EYE AND VISION 



[CH. LVI. 



The three axes around which the movements occur are : 



1. The visual or antero-posterior axis. (A P, fig. 595). 



2. The transverse axis, which connects the points of rotation of 

 the two eyes. (Tr, fig. 595). 



3. The vertical axis, which passes at right angles to the other 

 two axes through their point of intersection. 



The line which connects the fixed point in the outer world at 

 which the eye is looking to the point of rotation is called the visual 

 line. The plane which passes through the two visual lines is called 

 the visual plane. 



The various positions of the eyeballs are designated primary, 

 secondary, and tertiary. 



The primary position occurs when both eyes are parallel, the 

 visual lines being horizontal (as in looking at the horizon). 



Secondary positions are of two kinds : 



(1) The visual lines are parallel, but directed either upwards or 



Fir,. ,596. Identical points of the retinae. 



downwards from the horizontal (as in looking at the sky). 



(2) The visual lines are horizontal, but converge towards one 

 another (as in looking at a small object near to and immediately on 

 the same level as the eyes). 



Tertiary positions are those in which the visual lines are not 

 horizontal, and converge towards one another (as in looking at the 

 tip of the nose). 



It is possible to conceive positions of the eyeballs in which the 

 visual lines diverge from one another; but such positions do not 

 occur in normal vision in man. 



Both eyes are moved simultaneously, even if one of them 

 happens to be blind. They are moved so that the object in the 

 outer world is focussed on the two yellow spots, or other corre- 

 sponding points of the two retinae. The images which do not fall 

 on corresponding points are seen double, but these are to a great 

 extent disregraded by the brain, which pays particular attention to 

 those images which fall on corresponding points, 



