136 FIELD OF SIGHT. [BOOK in. 



outside the field of sight of that eye ; it has to be brought much 

 further forward until it comes into the field of sight of the left 

 eye ; the profile of the face and especially of the nose prevent 



Upper 



Nasal I i^fe' ;j Temporal 



Lower 



FIG. 152. THE VISUAL FIELD OF THE RIGHT EYE. (Anbert). 



The figure represents the visual field projected into space and therefore corresponds 

 to the objective field of sight ; the temporal side of the figure corresponds to 

 the nasal side of the retina. The shaded part indicates the increase gained 

 by looking outwards towards the temporal side, f, fovea ; x, blind spot. 



the rays reflected from the hand gaining access to the left 

 retina until the hand is brought a certain distance forward. 

 The right-hand side of the objective field of sight of the right 

 eye, corresponding to the nasal side of the retina of that eye, 

 extends much farther to the right than does the right-hand 

 side of the field of sight of the left eye, which corresponds to 

 the temporal side of the retina of that eye. Cf. Fig. 153. 

 Similarly, the left hand side of the field of sight of the left eye 

 extends farther to the left than does that of the right eye. 

 Hence on the one hand the total field of sight of the two eyes 

 together is increased in the horizontal diameter, subtending on an 

 average an angle of 180 instead of 145 ; and on the other hand 

 while a certain right-hand and left-hand part of the united fields 

 of sight belong respectively to the right and left eye only, the 

 remainder of the field is common to the two eyes. The area 

 common to the two eyes when the visual axes converge to the 

 same fixed point, is shewn as the shaded part in Fig. 153. 

 Rays of light from objects in the common part affect the 

 retinas of both eyes at the same time, vision is here binocular ; 

 rays of light from objects at the extreme right and left 

 affect only the right and left retina respectively, vision in 

 these parts of each eye is never binocular, always monocular. 

 The amount of each retina which is thus cut off from binocular 

 vision is determined by the prominence of the nose and profile 



