[II 



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405 



)ints of the Ibveae ; act, and bb are corresponding points of the 

 two eyes, because they are equidistant from c and lie in the same 

 meridians. The circle passing through the nodal points nn and 

 G .represents the horopter, because each of its points forms an 

 image on identical points of the two retinae ; point A at act, point 

 B at bb. The horopter has the same form in asymmetrical 

 secondary positions of the eyes, i.e. when the point fixated lies 

 outside the median plane. If in Fig. 190 

 the point fixed is not C but A, and the 

 lines Aa fall on the central points of the 

 foveae, then points C and B will fall on 

 corresponding points of the retina, and 

 points A, B, C and all others along the 

 line of the circle form the horopter. 



(c) When the eyes are in an asym- 

 metrical tertiary position (which, as we 

 have seen, is very rarely the case), the 

 horopter is represented in space by a 

 complex curve of double curvature 

 which passes through the nodal points 

 of both eyes (Helmholtz). 



V. We have seen that when the 

 images fall on the two retinae of the 

 double eye at disparate or non-corre- 

 sponding points there is double vision. 

 This is the more obvious the greater the 

 incongruence or the distance between 

 the points on which the two images fall 

 from those at which they should form a 

 single image. Homonymous and crossed 

 double images must be distinguished. 



In Fig. 191 a, b, c represent three 

 points in the median plane of the double 

 eye (L., R.}. On fixating point b, a single, 

 clear image is formed in the two foveae 

 b 1 , b z , while at the same time points a 

 aijd c form double images at non -corre- 

 sponding points a 1 a 2 , c 1 c 2 . As the 

 double eye is not focussed to the distances a and c, the double 

 images appear blurred; as point a is more distant and point c 

 nearer than the fixation point b, the double images of the former 

 appear smaller and those of the latter larger than b. The two 

 images of a are homonymous; that on the right disappears on 

 closing the right eye, and that on the left on closing the left eye. 

 The two images of c, on the contrary, are crossed ; that on the 

 right vanishes on closing the left eye, and that on the left on 

 closing the right eye. The place to which we refer the double 



PIG. 191. Geometrical construction 

 to show the production of homo-_ 

 lateral and crossed images. 



