SPA TIAL PER CEPTION. 1137 



because movements of convergence and accommodation were necessary 

 in order to see it distinctly. This view is altogether negatived by 

 experiments, and there is at present no evidence to show that move- 

 ment factors take any part whatever in relative localisation, while they 

 seem to be comparatively unimportant in the process of absolute 

 localisation in adult life. 1 The importance of binocular vision in spatial 

 perception was first shown by Wheatstone, who, however, adopted a 

 psychological view of the binocular mechanism, supposing that the 

 estimation of distance depended on a mental comparison of the two dis- 

 similar images of the two eyes, and this mode of describing the binocular 

 mechanism in psychological terms is still commonly adopted. 



Local signs. This term, which is due to Lotze, is often used in 

 connection with this subject. Lotze supposed that in the skin differ- 

 ences of quality of the sensation arising from stimulation of different 

 parts, were the basis of localisation, the differences of quality being pro- 

 duced by differences of texture and thickness of the skin and of under- 

 lying parts, and these differences of quality were spoken of as " local 

 signs." In the case of the retina Lotze supposed that the local signs 

 were movements or rather impulses to movement. He applied the 

 idea, however, only to estimation of height and breadth, and not of 

 depth, referring the latter to more psychological factors. When a 

 definite point of the retina is stimulated, a reflex movement occurs 

 which brings the image of the stimulating object on the place of most 

 distinct vision. Each retinal point will then become associated with a 

 definite movement, and Lotze supposed that the starting of this definite 

 movement was the local sign in the case of the retina, and acted in 

 the same way as the difference of quality in touch as the basis of 

 localisation. 



Bering's theory of the binocular mechanism. The most satis- 

 factory physiological theory of the binocular mechanism is, that 

 relative depth-perception depends on double images, or rather on 

 retinal disparation. This view was first advanced by Panum, 2 and has 

 been most fully developed by Hering. 3 According to this view, it is 

 held that retinal disparation is the physiological basis of the idea of 

 nearer or farther than the fixation point, the difference depending on 

 whether the disparation is crossed or uncrossed. An object is seen nearer 

 than the fixation point when it stimulates points with crossed dispara- 

 tion. The appearance of nearness occurs both when double images are seen 

 and when the disparation is so slight that the double images are fused into 

 one. Further, the greater the disparation, the greater is the apparent dis- 

 tance of the object from the fixation point. In the same way, uncrossed 

 retinal disparation acts as the basis of the idea of farther than the fixation 

 point. It must be clearly understood that our ideas of distance are not 

 held to depend on double images, which, as a matter of fact, few people 

 ever see, or probably ever have seen; but retinal disparation is held 

 to be the physiological basis of our ideas of relative distance in much the 

 same way that certain chemical changes in the retina are held to be the 

 physiological basis of our ideas of colour ; in both cases there is, of course, 



1 To what extent they take part in the genesis of spatial perception is another matter 

 which cannot be considered here. 



2 Arch. f. Anat., Physiol. u. wissensch. Me.d., 1861, S. 63 and 178; and "Physiol. 

 Untersuch. iiber das Sehen mit zwei Augen.," 1858. 



3 "Beitr. z. Physiol.," 1864, Heft 5 ; and Arch. f. Anat., Physiol. u. wissensch. Med., 

 1865, S. 152. 



VOL. II. 72 



