364 SIR DAVID BREWSTER ON THE LAW OF VISIBLE POSITION 



account for this property of the eyes have been unsuccessful, and that it must be 

 either a primary law of our constitution, or the consequence of some more general 

 law which is not yet discovered." This doctrine has been very generally admitted ; 

 and if great names could have given it currency, those of NEWTON and WOLLAS- 

 TON, supported by a number of anatomists and metaphysicians, might have placed 

 it, both optically and metaphysically, beyond the reach of challenge. The doc- 

 trine of the semidecussation of the fibres of the optic nerve, as explained by NEW- 

 TON, gave great support to the theory of corresponding points. The idea that 

 each fibre of the nerve divided itself into two, one of which 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 

 doctrine. 



Whether the anatomical supposition be true or false is a matter of little con- 

 sequence 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 be- 

 cause it is a necessary consequence of the general law of visible direction. 



Along with the theory of corresponding points, we must rank the binocular 

 circle of the Germans in which it is embodied. Let R, L, fig. 18, be the right and 

 left eyes whose centres of visible direction are C, C', and whose optic axes CA, C'A, 

 converge to any point A. Through the three points A, C, C', describe the circle 

 ABCC'. This circle is called the Binocular Circle, because if we take any point 

 B in its circumference, and draw BCE, BC'E', the points E, E' on the retinse will 

 be corresponding points, that is, points equidistant from D (because the angles 

 ACB, AC'B being equal, DC'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 farther from D in the left eye 

 than in the right eye ; that is, if the ray BR is refracted to E, the ray BL will 

 be refracted to some point e, and consequently the lines of visible direction EC, e C 

 will meet in a point without the circle ABC. The real binocular curve, therefore, 

 is everywhere without the circle. Hence the doctrine of corresponding points is 

 not true ; and if it had been true, it would have been so because it was a neces- 

 sary consequence of a law of visible direction. 



7. On the Vision of Cameos and Intaglios. 



The beautiful experiment of converting a cameo into an intaglio, and an in- 

 taglio into a cameo, by monocular vision, is well known. In 1825 I had occasion 



