1867.] 



Mr. A. Claudet on Binocular Vision. 



427 



easily satisfy ourselves that it is so ; for, looking with a single eye in both 

 cases, all the letters appear on the same plane, notwithstanding the differ- 

 ent distances of the two surfaces given by the position of the knots : and 

 we may add another convincing proof, which is that the pseudoscope in- 

 verts the distances of the surfaces. 



At first it is rather difficult to understand how the phenomenon can take 

 place ; for as the perception of the two surfaces is simultaneous, how is it 

 possible that during such a rapid revolution the optic axes can be made 

 to converge alternately on each surface while it is passing so quickly, and 

 that they should be made suddenly to converge on the other in its turn ? 



However, there cannot be any question that in reality the phenomenon 

 takes place, and that it is decidedly an effect of binocular vision ; therefore 

 it only remains to be explained how it can be produced. In endeavouring 

 to arrive at the true cause of the phenomenon, we shall have to bring to 

 mind various physiological sensations which concur in producing the effect. 

 One is the effort we make to obtain distinct vision, and the other the 

 effort -we make to obtain single vision. These two efforts act in unison ; 

 for it is impossible not to admit that the two muscular processes by which 

 both the angle of convergence is directed to the object and the focus of 

 the eyes is adapted to its distance, for the double purpose of having at 

 once single and distinct vision of every object, are two actions neces- 

 sarily simultaneous and inseparably connected. They are therefore both, 

 each in its way, criteria of the distances of objects ; but they give rise to 

 certain indirect and additional criteria for other distances, in two ways : one, 

 the most important, is the double images of the objects situated before and 

 behind the point of convergence ; and the other, but only in a subsidiary 

 way, the degree of confusion of the objects situated before and behind the 

 point of convergence and which are not in focus. 



The comparison of two points, one of which is in focus and well defined, and 

 the other out of focus and confused, helps considerably in forming a judgment 

 that they are on different planes. But in a question of binocular vision, per- 

 haps we ought not strictly to take into account this last criterion, which be- 

 longs equally to monocular and binocular vision ; and if we allude to it, it 

 is only because, although it does not produce the real stereoscopic effect, still 

 it contributes to give that sort of illusion of relief which by various means 

 may be evinced by monocular vision. Therefore it is particularly the sen- 

 sation of double images, the degree of their separation, and their respective 

 positions either outside or inside from the centres of the two retinae, which 

 indicate more powerfully the exact distance of the object from the point 

 of single vision either before or behind. 



"When we look fixedly on a point of one surface of the revolving card, 

 that point appears single, and we see at the same time another point on 

 the other surface which appears double, although we hardly feel that we 

 notice its doubleness; and from the position or distribution of the double 

 images, either on the right or on the left of the central point, we have at 



