412 PHYSIOLOGY CHAP. 



vision (as above j. Experience, moreover, shows that the two 

 images L and R of the pyramid fuse into the single image of a 

 body in three dimensions. This happens when the two diagrams 

 are examined through the stereoscope. 



The physiological explanation of this phenomenon given by 

 Brucke (1841), Prevost (1843), and Brewster (1857) is as follows. 

 In binocular vision the accommodation system of both eyes is 

 constantly altering; convergence and accommodation vary in- 

 cessantly, so that the images of the several sections of the pyramid 

 fall on identical points of both retinae in quick succession. This 

 constant variation must be regarded as the fundamental condition 

 of binocular tri-dimensioual vision. It is as if the eyes explored 

 the different points of the objects more or less distant from us, as 

 by touch we explore the objects around us. 



This ingenious interpretation is, however, contradicted by a 

 series of experiments made by Dove (1853), Aubert (1864), 

 Bonders (1867), and Bourdon (1902). To oi.tain a single, solid, 

 stereoscopic image of the two perspective figures L and R, of the 

 pyramid, only the shortest possible illumination, as an electric 

 spark, is required. It is impossible to assume that during such 

 instantaneous illumination there can be any movement of 

 accommodation, or exploring with the eyes (visual touch). 



This experiment leads us to ascribe less significance to the 

 absolute correspondence of the retinal points, and to conclude that 

 points in the two retinae which correspond approximately may 

 function together. It further leads us to conclude that the 

 correspondence of points is an acquired property, that is, determined 

 by habit. This view is confirmed by the fact that diplopia does not 

 occur in many cases of strabismus because a new set of correspond- 

 ing points are formed in the retina of the squinting eye. After 

 tenotomy the patients are often disturbed by the appearance of 

 double images for a few days immediately after the operation. It 

 is only the relative and acquired character of the corresponding 

 points that enables us to give any physiological explanation of the 

 stereoscopic fusion of two different perspective images that cannot 

 be superposed, even if we do not entirely exclude Briicke's 

 hypothesis of "visual touch" or the intervention of psychical acts 

 of perception and representation. 



Brucke himself fell back later on a psychological explanation 

 (necessarily vague and ill-defined) in which he assumed that it 

 is the task of the brain as organ of the mind to complete and 

 perfect the defects of direct visual sensations and endow them 

 with spatial representation. 



VIII. The most conclusive proof that binocular tri-dimensional 

 vision depends on the perspective differences between the two 

 uniocular images was given by Wheatstone in 1830 through the 

 discovery of the stereoscope. As shown in the diagram (Fig. 198) 



