138 



January S, 1852. 

 COLONEL SABINE, R.A., V.P. and Treas., in the Chair. 



Charles "Wheatstone Esq., F.R.S., delivered the Bakerian Lecture, 

 being the substance of his paper entitled, " Contributions to the 

 Physiology of Vision. — Part II. On some remarkable, and hitherto 

 unobserved, phaenomena of Binocular vision, — (continued)." Re- 

 ceived January S. 



The lirst part of these researches was communicated to the Royal 

 Society in 1838, and published in the Philosophical Transactions for 

 that year. 



The second part, nov presented, commences -^'ith an account of 

 some remarkable illusions vrhich occur when the usual relations 

 which subsist between the magnitude of the pictures on the retinas 

 and the degree of inclination of the optic axes are disturbed. L'nder 

 the ordinary circumstances of vision, when an object changes its 

 distance from the observer, the magnitude of the pictures on the re- 

 tinte increases at the same time that the inclination of the optic axes 

 becomes greater, and vice versa, and the perceived magnitude of the 

 object remains the same. The author wished to ascertain what 

 would take place by causing the optic axes to assume every degree 

 of convergence while the magnitude of the pictures on the retinte 

 remained the same ; and, on the other hand, the phenomena which 

 would be exhibited by maintaining the incHnation of the optic axes 

 constant while the magnitude of the pictures on the retinae continu- 

 ally changed. To effect these purposes, he constructed a modification 

 of his reflecting stereoscope ; in tliis instrument two similar pictures 

 are placed, on moveable arms, each opposite its respective mirror; 

 these arms move round a common centre in such manner, that, how- 

 ever they are placed, the reflected images of each picture in the mir- 

 rors remains constantly at the same distance fi'om the eye by which 

 it is viewed ; the pictures are also capable of sliding along these arms, 

 so that they may be simultaneously brought nearer to, or removed 

 further from, the mirrors. Yv'hen the pictures remain at the same 

 distance and the arms are removed round then- centre, the reflected 

 images, vrhile their distances from the eyes remain unchanged, are 

 displaced, so tlmt a diflerent inclination of the optic axes is required 

 to cause them to coincide, When the arms remain in the same 

 positions and the pictures are ])rought simultaneously nearer the 

 mirrors, the reflected images are not displaced, and they always co- 

 incide with the same convergence of the optic axes ; but the mag- 

 nitude of the pictures on the retinae becomes greater as the pictures 

 approach. The experimental results aflbrded by tliis apparatus, so 

 far as regards the perception of magnitude, are the following ; the 

 pictures being placed at such distances, and the ai"ms moved to such 

 positions, tliat the binocular image appears of its natural magnitude 

 and its proper distance, on the arms being moved so as to occasion 

 the optic axes to converge less, the image appears larger, and ou 

 their being moved so as to cause the optic axes to converge more, 



