VIII 



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405 



|iui!iis of the 1'oveae; at* and bb are corresponding points of the 

 two eyes, because they are r(|iii<lisiant from c and lie in tin- same 

 meridians. The circle passing through the nodal points mi and 

 C represents the horopter, because each of its points fun us an 

 image mi identical points of the two retinae; point A at aa, point 

 at bb. The horopter has the same form in asymmetrical 

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

 outside the median plane. If in Fig. 190 

 the point lixed 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, L', 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. Homonijmous and crossed 

 double image* 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-, while at the same time points a 



' 111- ''" 191. Geometrical construction 



and c form double images at non-corre- to show the production of homo- 



j- i .< i a i ,1 1 it'-iul ,-iMil i-rnsst'il i!ii:t'_'i's. 



spending points a 1 a-, c 1 C L . 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 Litter larger than b. The two 

 i m ages of a are homonymoua ; 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 



