782 VISUAL SENSATIONS. [BOOK in. 



tion of the eye, the rays of light proceeding from a portion 

 only of the external world fall upon the retina ; or in other 

 words in any one position of the eye only a portion of the 

 external world is visible at the same time. The portion so 

 seen is spoken of as the visual field for that position. 



The image thrown on the retina is an inverted one, so that the 

 top of an actual object is represented by the lower, and the bot- 

 tom by the upper part of the retinal image ; similarly the actual 

 left-hand side of the retinal image corresponds to the right- 

 hand side of the actual object, and the right-hand side to the 

 left-hand side. Hence the right-hand half of the visual field 

 corresponds to the left-hand side of the retina, and the left-hand 

 half to the right-hand side. 



The eye can be moved in various directions, and since in the 

 visual field the portion of external nature which can be seen at 

 the same time differs with each different position, a large range 

 of vision is thus secured; and this can be further increased by 

 movements of the head. Moreover we normally make use of 

 two eyes, our normal vision is binocular ; and the visual field 

 of the right eye differs from that of the left eye. There is one 

 striking difference which must always be borne in mind. A 

 section carried through the eye in a vertical and front-to-back 

 plane, through what we shall learn to call the optic axis (Fig. 

 138, ox) (the exact details of the plane may be left for the 

 present), will divide the retina into two lateral halves, and in 

 each retina one half will be on the nasal side next to the nose, 

 and the other half will be on the malar or temporal side, 

 next to the cheek or temple. It must be remembered that the 

 nasal halves and temporal halves of the two retinas do not 

 occupy corresponding positions in space. The temporal half of 

 the left retina is on the left side of its own eye, whereas the tem- 

 poral half on the right retina is not on the left but on the right 

 side of its eye ; and so with the nasal halves. Now in the right 

 eye, the right-hand side of the visual field corresponds to the 

 nasal half of the retina, and the lefVhand side of the visual 

 field to the temporal half of the retina, whereas in the left eye 

 the right-hand side of the visual field corresponds to the tem- 

 poral half of the retina, and the left-hand side to the nasal half. 

 This is shewn in Fig. 138, where the left-hand visual field and 

 the retinal area concerned are shewn shaded in each eye. 



When we look at an object with the two eyes, though two 

 retinal images are produced, one in one eye and one in the other, 

 we perceive one object only, not two. This is the essential fact 

 of binocular vision ; when certain parts cf each retina are stimu- 

 lated at the same time we are conscious of one sensation only, 

 not two ; and the parts of the two retinas which, stimulated at 

 the same time, give rise to one sensation are spoken of as " cor- 

 responding parts." From the structure and relations of the 



