542 



BINOCULAR VISION. 



[BOOK in. 



Hence a point lying anywhere on the right side of one retina, has 

 its corresponding point on the right side of the other retina, and 

 the points on the left of one correspond with those on the left of 



FIG. 74. DIAGRAM ILLUSTRATING CORRESPONDING POINTS. 



L the left, E the right eye, K the optical centre, a v &,, C L are points in the 

 right eye corresponding to the points a, b, c in the left eye. The two figures below 

 are projections of L the left and E the right retina. It will be seen that a on the 

 malar side of L corresponds to lf on the nasal side of R. 



the other. Thus, while the upper half of the retina of the left eye 

 corresponds to the upper half of the retina of the right eye, and the 

 lower to the lower, the nasal side of the left eye corresponds with 

 the malar side of the right, and the malar of the left with the 

 nasal side of the right. 



The blending of the two sensations into one only occurs when 

 the two images of an object fall on these corresponding points of 

 the two retinas. Hence it is obvious that in single vision with two 

 eyes the ordinary movements of the eyeballs must be such as to 

 bring the visual axes to converge at the object so that the 

 two images may fall on corresponding points. When the visual 

 axes do not so converge, and when therefore the images do not fall 

 on corresponding points, the two sensations are not blended into 

 one perception and vision becomes double. 



Movements of the Eyeballs. 



The eye is virtually a ball placed in a socket, the bulb and the 

 orbit forming a ball and socket-joint. In its socket-joint the optic 

 ball is capable of a variety of movements, but it cannot by any 

 voluntary effort be moved out of its socket, It is stated that by a 

 very forcible opening of the eyelids the eyeball may be slightly 



