CHAP. HI.] 



SIGHT. 



941 



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 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 



Upper 



Nasal 



Temporal 



Lower 



Fio. 156. THE TISUAL FIELD OF THE RIGHT ETE. (Aubert.) 



The figure represents the visual field projected into space and therefore cor- 

 responds to the objective field of sight ; the temporal side of the figure corre- 

 sponds to the nasal side of the retina. The shaded part indicates the increase 

 gained by looking outwards towards the temporal side. /, fovea ; z, blind spot. 



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. 157. Simi- 

 larly, 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. 157. 

 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 between the eyes ; in 

 some of the lower animals the position of the eyes is so com- 

 pletely lateral that no rays of light proceeding from the same 



