OPTICS 281 



4. Outward (r. inf.) by lowering (o. sup.) and adduction 

 (r. int.). 



2. Combined action of the muscles of both eyes. The 

 two eyes are moved simultaneously. They are innervated 

 from a common centre. The movements are as follows : 



1 . Rect. sup. and obi. inf. on both sides = raising of both 

 eyes; 



2. Rect. inf. and obi. sup. on both sides = lowering of 

 both eyes ; 



3. The left rect. int. and right rect. ext. = movement of 

 both eyes to the right; 



4. The left rect. ext. and right rect. int. = movement of 

 both eyes to the left. 



5. Rect. int. on both sides convergence; 



6. Rect. ext. on both sides = divergence. 



With the convergence is associated accommodation and con- 

 striction of pupil. 



Binocular point of vision is the point in space on which 

 both eyes are fixed and in which, therefore, the two visual 

 axes meet. 



Binocular 'field of rotation is the field which includes all the 

 object points which can be perceived by the two eyes when 

 the head is held stationary. 



The monocular fields of the two eyes nearly but not altogether 

 cover each other. But the binocular field of vision is much smaller 

 than that part of the monocular fields common to both eyes, for 

 the two visual axes cannot be directed simultaneously upon a point 

 upon which each visual axis can independently be directed. 



III. Binocular vision. 



i . Single vision with both eyes. Those objects in the 

 outer world whose images fall on identical points of both 

 retina; are seen as single objects. Identical points of the 

 two retinas are therefore such points whose simultaneous 

 stimulation by a luminous object gives rise to a single sen- 

 sation. 



A pair of identical points are, for example, the two foveae 

 centralis, and also two points on both retinae equidistant and 



