CHAPTEE VIII 



OCULAR MOVEMENTS AND VISUAL PERCEPTIONS 



COXTKXTS. 1. Articulation of eye-ball in its socket : its external muscles ; its 

 movements and possible positions. '2. Isolated and associated movements of its 

 muscles. 3. The innervation and co-ordination of the eye-movements. 4. Simple 

 binocular vision and tin- horopter. f>. Diplopia. 6. Conflict between the visual 

 images of both eyes and the phenomena of binocular contrast. 7. Spatial 

 perception in monocular and binocular vision. 8. Stereoscopic binocular vision ; 

 the stereoscope. 9. Psychophysical processes on which visual perceptions and 

 representations depend : relativity of our judgments of size, distance and 

 form ; optical illusions and visual hallucinations. 10. Protective apparatus of 

 the eye. 11. Origin of t lie aqueous humour. Bibliography. 



IN the last chapter we discussed the simplest and most elementary 

 characteristics of Vision, including the sensations' of light and 

 colour produced by stimulation of the elements of the retina, 

 apart from the manifold and varied sensations that are simul- 

 taneously aroused at the different points of the retinal surface. 



On this complex of sensations depend our visual perceptions, 

 that is, the knowledge we acquire of external objects through 

 the central organs of vision. The impression commonly made 

 upon us by the visible world is not that of a multitude of separate 

 and independent elementary sensations, but rather of a greater 

 or less number of compound sensations, which we project outwards 

 by means of psychical processes of synthesis and association, and 

 transform into perceptions of the objects, judging of their bright- 

 ness and colour, their form, size, and distance, and the nature and 

 velocity of their movements. 



Till now, moreover, we have only been considering uniocular 

 vision, whereas our ordinary sight is binocular, and is dependent 

 on the great mobility of the eye-balls, and the associated and 

 variously co-ordinated activity of their external muscles. In 

 uniocular vision objects are viewed almost entirely in one plane. 

 Their distance can only be appreciated by the effort of accommoda- 

 tion, which does not come into play for long distances. Uuiocular 

 vision is therefore imperfect. Binocular vision gives us the 

 following advantages : 



(a) Greater extension of the visual field ; 



388 



