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PHYSIOLOGY 



CHAP. 



ence " of the two retinae, into which we need not enter as it has 

 little practical importance. 



Passing from the centre to the periphery of the retina, the deter- 

 mination of corresponding points becomes increasingly uncertain, 

 especially in the lateral direction (Mandelstamm and Scholer). 



The simplest case of binocular vision is that in which the two 

 eyes are in the primary position, with the two visual axes parallel 

 and in a horizontal plane. On fixating a distant point on the 

 horizon or a star in the sky we see singly not only the point or 

 star which form images in the centre of the two foveae (direct 

 vision), but also the surrounding points or stars which form images 

 more or less removed from the centre of the" foveae (indirect 



vision). Johannes Miiller gave 

 the name of horopter to the geo- 

 metrical figure which results from 

 the spatial points that are seen 

 singly with the double eye, and 

 therefore form images on corre- 

 sponding points of the two 

 retinae. 



All spatial points not included 

 in the figure of the horopter 

 appear double when we direct our 

 attention upon them, the more so 

 in proportion as they are removed 

 from the horopter, because their 

 images fall on disparate points of 

 the retina. 



Each position of the binocular 

 apparatus has its corresponding 

 horopter figure. 



(a) When the eyes are in the primary position, the horopter 

 would be represented by a vertical plane situated at infinite 

 distance, if the vertical axes of the two retinae were exactly 

 parallel. Actually, however, this is not the case: if the visual 

 axes of the eyes are set for vision at a long distance, the vertical 

 axes converge somewhat below, so that, in the erect position of 

 the head, they intersect at about the level of the plane of the 

 feet (Helmholtz). It follows that not only the points of the 

 horizon, but also those on the plane on which we stand, are seen in 

 single vision an obvious advantage to vision as a whole. 



(6) When the eyes are in a secondary position, i.e. when the 

 visual axes converge and lie in the horizontal plane, the horopter 

 is formed like a circle passing through the nodal points of both 

 eyes and the point of convergence of both visual axes (Vieth and 

 Joh. Miiller). In Fig. 190, C is the convergence point of the 

 two visual axes along the median plane MM\ cc are the central 



FIG. 190. Horopter circle. (Joh. Miiller.) 



