viii OCULAR MOVEMENTS 403 



nasal side of the right retina, extends much farther to the right 

 than the inner part of the visual field of the left eye, correspond- 

 ing with the temporal side of the left retina. Similarly, the outer 

 limit of the visual field of the left eye extends farther to the left 

 than the inner limit of the visual field of the right eye. The field 

 common to both eyes, on convergence of the two visual axes at 

 the point F y is represented by the shaded area, and in this only 

 is there vision with the "double eye." In many animals the 

 position of the eyes lies so much to the side that the two fields of 

 vision do not intersect, and consequently they have only uniocular 

 vision. 



The blind spot of the left visual field falls, in the binocular 

 field of vision, on a seeing part of the right visual field, and vice 

 versa, so that there is no physiological scotoma when we employ 

 binocular vision. 



In order to have single vision in the binocular visual field it 

 is necessary for the images of the object to fall on certain points 

 of the two retinae, which are 

 known as corresponding or iden- 

 tical points. When this funda- 

 mental condition is not fulfilled, 

 i.e. when the images of the object 

 fall on disparate non-correspond- 

 ing points of the two retinae, 



diplopia Or double vision Of the FlG . 189 ._ Diagram O f identical or correspond- 

 Object results in S P ints in tne two retinae. L, left eye ; 



J ,, ., , . R, right eye ; c, centre of fovea. 



What are the corresponding 



points, i.e. the points in the two retinae, which, when excited 

 simultaneously, produce single vision that is, vision of the image 

 in the same part of the binocular field of vision ? 



It is plain that the centres of the two foveae are identical 

 points, because on focussing any point with the double eye both 

 visual axes converge upon it. If we picture the two retinae as 

 perfect sections of spheres the poles of which correspond with the 

 centres of the foveae, and divide each retina into four parts by 

 lines in the vertical and horizontal meridians intersecting at a 

 right angle at the centre of each fovea (Fig. 189), we may con- 

 sider that not only the poles (c) but also all corresponding points' 

 in the four quadrants (1, 2, 3, 4) of the two retinae would be 

 identical if the two retinae were superposed. Or better, we may 

 say, with Johannes Miiller, that not only the central points of the 

 two foveae, but all points that are equidistant in the same 

 meridian from the centre of the foveae, are identical. 



This geometrical definition of the corresponding points cannot, 

 however, be regarded as exact, because the four segments of the 

 two retinae are not fully congruent. Later investigation with 

 more perfect methods showed a slight "physiological incongru- 



