Mr. x\. Claudet on Binocular Vision. 



553 



feel that we notice its doubleness ; and from the position or distri- 

 bution of the double images, either on the right or on the left of the 

 central point, we have at the same glance the perception of the re- 

 spective distances. Therefore, to judge of the distances of certain 

 objects in the direction of the line of vision, we are not absolutely 

 obliged to alter constantly the angle of convergence. This is proved 

 by our perception of the two distances of the surfaces of the card 

 while it is revolving ; for it would be impossible that we should alter 

 the angle of convergence to adapt it alternately to the two surfaces 

 while they are turning so rapidly. 



The same angle of convergence kept on one or the other surface 

 is no impediment to our seeing both in a sufficiently distinct manner. 



The whole phenomenon may be better understood by the illus- 

 tration given in fig. 6. 



When we converge the optic axes on B, 

 this point, being represented on the centre 

 of both retinae at B' B", is single, but A 

 being nearer is represented on the left of 

 the centre of the left retina at A', and on 

 the right of the centre of the right retina 

 at A"; therefore it appears double. 



For the same reason, converging on A, 

 this point is single, but B is double, with 

 this difference — that one image is on the 

 right of the left retina, and the other on 

 the left of the right retina ; so that the 

 double images of nearer objects situated at 

 A are represented outside the centres of 

 the two retinse, and those of further ob- 

 jects situated at B are represented inside 



the centres of the retinse, and each of these two different sensations 

 brings to our mind the perception of the distance which has produced 

 it. During the revolution of the card we may adapt the convergence 

 either to one or to the other surface and keep it so ; but in every 

 case the letters on that surface will appear single and a little better 

 defined ; and this, with the sensation of double images of the letters 

 on the other surface, will be an indication of their respective dis- 

 tances. 



As I am not aware that the illusion I have described in this paper 

 has ever been noticed before, it has appeared to me that its publica- 

 tion would excite the interest of all those who look for any new fact 

 capable of illustrating the principles of binocular vision, and showing 

 the wonderful property of the angle of convergence, by which the 

 most minute differences in the distances of objects and the slightest 

 relief on their surfaces can be detected, and by which also in the 

 abnormal conversion introduced in its action by the pseudoscope all 

 our sensations are reversed. Therefore the pseudoscope is the great 

 test of the phenomena of binocular vision ; for by reversing certain 

 sensations which by constant habit we may hardly notice, it ren- 

 ders them more conspicuous by the comparison of the abnormal state 



