Prof. Wheatstone on the Physiology of Vision. 263 



R and L (fig. 26) are the two eyes ; CA, C'A the optic axes 

 converging to the point A ; and CABC is a circle drawn through 

 the point of convergence A and the centres of visible direction 

 CC. If any point be taken in the circumference of this circle, 

 and lines be drawn from it through the centres of the two eyes 

 CC, these lines will fall on corresponding points of the two 

 retina: DD' ; for the angles ACB, AC'B being equal, the angles 

 DCE, DC'E are also equal ; therefore any point placed in the 

 circumference of the circle CABC will, according to the hypo- 

 thesis, appear single while the optic axes are directed to A, or 

 to any other point in it. 



I will mention two other properties of this binocular circle : 

 1st. The arc subtended by two points on its circumference con- 

 tains double the number of degrees of the arc subtended by the 

 pictures of these points on either retina, so that objects which 

 occupy 18CP of the supposed circle of single vision are painted 

 on a portion of the retina extended over 90° only ; for the angle 

 DCE or DCE being at the centre, and the angle BCA or BCA 

 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 CAC CBC 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 sim- 

 plify the consideration of the problem, R and L represent only 

 the circle of curvature of the bottom of the retina, but the rea- 

 soning 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 accurate expression of the phse- 

 nomena of single vision. According to the former, objects 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 witlj 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 niaintaincrs of the 

 theory of corresponding points, namely that the two pictures 

 projected by any object on the r< time are exactly similar, is quite 

 contrary to fact in every COM except that in which the optic axes 



are parallel. 



GawendlU, Porta, Tacquet and Gall maintained, that we sec 



