J. LeConte — Phenomena of Binocular -Vision. 107 



This capacity is not unique, except in its extreme refinement. 

 The same is true of all bodily actions and sense-perceptions. 

 For example, the complex and delicate play of muscular action 

 in maintaining equilibrium in standing, the subtle causes or 

 signs of facial expression, often elude our utmost power of 

 analysis. Under the same head also comes the so-called muscle- 

 reading, only that this is possible in but few while face-reading 

 is practiced by all. In all such cases we are guided or judge by 

 subtle signs which escape conscious detection ; but we would 

 be wrong to conclude on that account that the signs are not 

 physical. 



(3.) Geometric construction in the manner of Brewster is use- 

 ful or even indispensable in representing binocular visual 

 phenomena, because it is simple and easily understood. But 

 as soon as we study carefully we find that this method does not 

 give truly the place of double images. (1) In the representa- 

 tion of binocular perspective of objects or points one beyond 

 the other it utterly fails, because when it tries to represent the 

 double images it does not represent the relative distance ; and 

 when it tries to represent the relative distances it does not re- 

 present the doubling of the images. It was to remedy this 

 defect that I proposed and put in practice a "new method of 

 diagrammatic representation."* This new method represents 

 truly all the phenomena of binocular perspective of objects 

 lying one bej^ond another in the line of sight, but cannot repre- 

 sent objects or points on a phantom plane. (2) In the phantom 

 image of a tessellated plane, a case for which it seems eminently 

 adapted, the ordinary geometric construction, as we have seen, 

 cannot truly represent the form of the surface on account of the 

 concavity of the retina. The law of corresponding points, how- 

 ever, completely explains it. (3) In the case of ocular divergence, 

 all diagrammatic representations of distance of course fail, for in 

 this case there is no point of optic convergence, and therefore 

 no point of sight. But even these cases are easily reducible to 

 the law of corresponding points. I drew attention to and ex- 

 plained these cases in 1875.* 



We repeat then that the law of corresponding points is by far 

 the most fundamental and general law of binocular vision. 

 But it is not necessary, in order to explain the oneness of the im- 

 pressions on corresponding rods or cones, to assume, as some do, 

 that these are two nerve-fiber-terminals of one brain cell. This 

 wonderful property is probably an acquired one ; not indeed 

 acquired by individual experience alone, but by the experience 

 of the whole line of vertebrate ancestry inherited and accumu- 

 lated. Its wonderful, almost mathematical exactness is the 

 result of the exquisite refinement of retinal structure and the 

 constant use of the eye in the accurate estimate of distance. 



*Thia Jour., vol. ix, p. 163, 1875 



