BINOCULAR VISION. 



1125 



first eye. The method most widely used is that of substitution, which 

 depends on identical localisation with the stereoscope or haploscope. 



In considering the mechanism of corresponding points, it is necessary 

 to have some definite terminology for different parts of the retina. It is 

 now usual to regard each retina as made up of a number of points 

 arranged in vertical and horizontal sections. One point corresponds with 

 another, when it lies in the same vertical and horizontal sections in the 

 two eyes. Points which do not correspond are termed " disparate " 

 points. If a point lies in the same horizontal section in the two eyes, 

 but in different vertical sections, the points are termed horizontally 

 disparate, or as having horizontal disparation. If they lie in the same 

 vertical but in different horizontal sections, they have vertical disparation. 



It has been found that the strictly vertical meridians of the two eyes 

 do not correspond with one another. If the double images of two lines 

 are observed, it is found that these appear to be vertical and parallel to 

 one another, when the lines diverge from one another upwards, so as to 

 form an angle of 2 with one another. This " physiological incongruence " 

 of the two retinae varies in amount in different individuals ; the deviation 

 of apparently vertical lines may reach 2 0- 30', but is much less and 

 may be absent in myopia. 



According to Volkmann, 1 there is a similar incongruence between the 

 horizontal meridians, amounting, however, only to half a degree, the 

 meridian of each eye inclining slightly downwards and outwards. 



Another kind of incongruence has been described by Kundt, 2 who 

 found that when he halved a horizontal line seen with one eye, the outer 

 half was made constantly too large, and Hering 3 supposes that this is 

 due to difference in the distance between corresponding points on the 

 two sides of the retina. 



Single and double vision. When retinal disparation is marked, 

 two images of an external point are seen. The nature of the dispara- 

 tion and diplopia depends on the position of the point in relation to the 

 fixation point. When nearer, the double images are crossed or homo- 

 nymous, i.e. the image of the right eye appears to the left, and that of 

 the left eye to the right ; when the point is beyond the fixation point, the 

 diplopia is uncrossed or heter- a ft e d 



onymous. The retinal dispara- i 



tion is similarly termed crossed 

 and uncrossed respectively. 



When the disparation is 

 slight, fusion of the two images 

 occurs, and single vision results. 

 This was first shown by Wheat- 

 stone, 4 and the amount of 

 disparation with which single 

 vision can occur has been in- 

 vestigated by Volkmann. 5 He 

 combined vertical lines (Fig. 

 400), in which a, &, and c were fixed, and d movable; a and I being pre- 

 sented in the stereoscope to the left, c and d to the right eye. He found 



1 "Physiol. Untersuch. im Gebiete d. Optik," Leipzig, 1864, S. 206. 



^Ann."d. Phys. u. Ohem., Leipzig, 1863, Bd. cxx. S. 118. 



3 Hermann's "Handbuch," Bd. iii. S. 362. *Phil. Trans., London, 1838, p. 371. 



5 Arch. f. Ophth., 1859, Bd. v. Abth. 2, S. 1. 



FIG. 400. 



