450 Sir David Brewster on the Lata of Visible Position 



went to a given point in the retina of one eye, while the other 

 went to the corresponding point in the retina of the other eye, 

 seemed to be at once an explanation and a proof of the doc- 

 trine. 



Whether the anatomical supposition be true or false is a 

 matter of little consequence at present, as the doctrine which 

 it supports is not true excepting in the single case where the 

 optic axes are parallel, and in this case it is true only because 

 it is a necessary consequence of the general law of visible di- 

 rection. 



Along with the theory of cor- 

 responding points, we must rank 

 the binocular circle of the Ger- 

 mans in which it is embodied. 

 Let R, L, fig. 18, be the right 

 and left eyes whose centres of 

 visible direction are C, 0, and 

 whose optic axes C A, A, 

 converge to any point A. 

 Through the three points A, C, 

 0, describe the circle A B C 0. 

 This circle is called the Bino- 

 cular Circle, because if we take 

 any point B in its circumference, 

 and draw B C E, B E', the 

 points E, E' on the retinae will 

 be corresponding points, that is, points equidistant from D 

 (because the angles A C B, A B being equal, D E' and 

 DCE are also equal), and consequently when the optic axes 

 are directed to A, an object at B will have its image formed 

 upon the corresponding points E, E', and will be seen single. 



Now, when the optic axes are directed to A, a ray from B 

 will fall upon the left eye at L with a greater angle of incidence 

 than on the right eye at R ; and consequently it will strike the 

 retina at a point further from D in the left eye than in the 

 right eye ; that is, if the ray B R is refracted to E, the ray 

 B L will be refracted to some point e, and consequently the 

 lines of visible direction EC, eC will meet in a point without 

 the circle ABC. The real binocular curve, therefore, is every- 

 where without the circle. Hence the doctrine of correspond- 

 ing points is not true; and if it had been true, it would have 

 been so because it was a necessary consequence of a law of 

 visible direction. 



B' JC 



