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XLIV. On Binocular Vision. By Dr. Wilhelm von Bezold*. 



THE author has recently published some articles on binocular 

 vision in the Zeitschrift fiir Biologie-\. The object of this 

 communication is to give a condensed account of their chief 

 contents. A few preliminary observations are here given, in 

 order to render the matter intelligible to the reader who is not 

 acquainted with the physiological literature on the subject. 



A theory of binocular vision must first of all satisfy two re- 

 quirements. On the one hand it must explain the occurrence 

 of a simple perception of solidity through the cooperation of 

 both eyes in vision ; on the other hand it must account for the 

 double images which are produced under certain circumstances. 



For the attainment of this object two methods have been 

 pursued. One was followed by J. Mtiller, and depends on the 

 principle which he propounded of the " Identity of the Retinas." 

 He says the points of the two retinas correspond in pairs in such 

 a manner that the excitation of two such corresponding (or, as 

 he thence calls them, "identical") points always produces a single 

 perception ; while the excitation of non-identical points always 

 produces a double image. But those points are identical on 

 which, with parallel visual lines (optic axes J), an infinitely 

 distant point is depicted. On viewing the heavens, for example, 

 the images of all stars fall upon identical points. 



If now, starting from this principle, we seek the geometrical 

 focus of the points which, for any position whatever of the fixed 

 point, are formed on identical points of the retina, and must 

 therefore appear single, we find in general a curve with several 

 branches, which is called the " mathematical Horopter" §. All 

 points which do not lie in it must, if the theory be correct, 

 appear double, and at a greater distance the further the two 

 images are from each other on the double retina. By " double 

 retina" is to be understood the combination of the two retinas 

 when they are supposed to be so laid one upon another that 

 identical points coincide. 



Now experience teaches that the apparent distance of the 

 double images is, in fact, proportional to the distance of cor- 

 responding points on the double retina, provided this distance 

 exceeds certain limits, the " limiting distances" [Grenzdistanzen). 

 If it is within these limits, the object appears single, even if it 

 does not lie in the mathematical horopter. Hence we speak of 



* Communicated by the Author, being an abstract of three Papers 

 contributed by him to Voit's Zeitschrift fur Biologic 



t Vol. i. p. 169-237 ; vol. ii. p. 178. 



% A visual line (Gesichtslinie), according to Helmholtz, is the straight 

 line which connects the fixed point with its image on the retina — that is, 

 with the fovea centralis. 



§ E. Hering, Beitmge zur Physiologie, parts 3, 4, 5. 



