VISION. 



809 



After-images. When a bright light is thrown on the eye and 

 then suddenly put out, there remains for a short time an impression 

 of the same light, as though the retinal molecules still continued to 

 vibrate from the light stimulus. This is a positive after-image. 

 When the eye has received a stimulus for some time, the sensation 

 which follows the withdrawal is of a different kind, and you have a 

 negative after-image, which is due to exhaustion of the retinal cells. 

 For instance, if you look at a red color for some time and the eye 

 afterward is focused on a white ground, the negative after-image is 

 a greenish-blue; that is, the color of the negative image is comple- 

 mentary to that of the object. 



Fig. 360. Muscles Associated in Moving the Eyeballs in the Directions 

 Indicated by the Arrows. (BALL.) 



Phosphenes. If the retina be pricked, compressed, or twitched 

 by any sudden movement, an impression of light will be produced. 

 The same effect follows the use of electricity. Hence the retina is 

 an essentially sensitive membrane. No matter by what cause its 

 sensibility be excited, it always gives rise to the subjective phe- 

 nomenon of a luminous sensation. 



Vision with Both Eyes. The study of phenomena bearing upon 

 this subject comprises: (1) movements of the eyes, (2) binocular vision, 

 and (3) the advantages of sight with loth eyes. 



MOVEMENTS OF THE BYES. The eyeball may be considered as 

 an articulated spherical globe which turns upon three axes that cross 

 each other. Six voluntary muscles affect the three rotations of the 

 eye. The rectus internus and externus, when acting alone, turn the 

 eye from side to side. The superior and inferior recti give to the 



