390 MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 



point A ; and C A B C is a circle drawn through the point of convergence A and 

 the centres of visible direction C C. If any point be taken in the circumference of 

 this circle, and lines be drawn from it through the centres of the two eyes C C, these 

 lines will fall on corresponding points of the two retinse D D' ; for the angles A C B, 

 A C B being equal, the angles D C E, D C E are also equal ; therefore any point 

 placed in the circumference of the circle C A B C will, according to the hypothesis, 

 appear single while the optic axes are directed to A, or any other part in it. 



I will mention two other properties of this binocular circle: 1st. The arc sub- 

 tended by two points on its circumference contains double the number of degrees 

 of the arc subtended by the pictures of these points on either retina, so that objects 

 which occupy 180° of the supposed circle of single vision are painted on a portion of 

 the retina extended over 90° only ; for the angle D C E or D C E being at the centre, 

 and the angle B C A or B C A at the circumference of a circle, this consequence 

 follows. 2ndly. To whatever point of the circumference of the circle the optic axes 

 be made to converge, they will form the same angle with each other ; for the angles 

 C A C, C B C are equal. 



In the eye itself, the centre of visible direction, or the point at which the principal 

 rays cross each other, is, according to Dr. Young and other eminent optical writers, 

 at the same time the centre of the spherical surface of the retina, and that of the 

 lesser spherical surface of the cornea ; in the diagram (fig. 26.), to simplify the con- 

 sideration of the problem, R and L represent only the circle of curvature of the 

 bottom of the retina, but the reasoning is equally true in both cases. 



The same reasons, founded on the experiments in this memoir, which disprove the 

 theory of Aguilonius, induce me to reject the law of corresponding points as an ac- 

 curate expression of the phenomena of single vision. According to the former, ob- 

 jects can appear single only in the plane of the horopter ; according to the latter, only 

 when they are in the circle of single vision ; both positions are inconsistent with the 

 binocular vision of objects in relief, the points of which they consist appearing single 

 though they are at different distances before the eyes. I have already proved that 

 the assumption made by all the maintainers of the theory of corresponding points, 

 namely that the two pictures projected by any object in the retinae are exactly similar, 

 is quite contrary to fact in every case except that in which the optic axes are parallel. 



Gassendus, Porta, Tacquet and Gall maintained, that we see with only one eye 

 at a time though both remain open, one according to them being relaxed and inat- 

 tentive to objects while the other is upon the stretch. It is a sufficient refutation of 

 this hypothesis, that we see an object double when one of the optic axes is displaced 

 either by squinting or by pressure on the eye-ball with the finger ; if we saw with 

 only one eye, one object only should under such circumstances be seen. Again, in 

 many cases which I have already explained, the simultaneous affection of the two re- 

 tinae excites a different idea in the mind to that consequent on either of the single 

 impressions, the latter giving rise to the idea of a representation on a plane surface, 



