552 Royal Society : — 



The cause of the anomaly resulting from the two different experi- 

 ments is entirely and positively due to a sensation of binocular 

 vision ; and we may 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 different distances of the two surfaces 

 given by the position of the knots : and we may add another con- 

 vincing proof, which is that the pseudoscope inverts the distances of 

 the surfaces. 



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

 take place ; for as the perception of tne 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 pass- 

 ing 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 pheno- 

 menon takes place, and that it is decidedly an effect of binocular 

 vision ; therefore it only remains to be explained how it can be pro- 

 duced. In endeavouring to arrive at the true cause of the pheno- 

 menon, we shall have to bring to mind various physiological sensa- 

 tions 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 necessarily simultaneous and inseparably connected. They 

 are therefore both, each in its way, criteria of the distances of ob- 

 jects ; 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 de- 

 fined, and the other out of focus and confused, helps considerably in 

 forming a judgment that they are on different planes. But in a ques- 

 tion of binocular vision, perhaps we ought not strictly to take into 

 account this last criterion, which belongs 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 w T hich by various means may be 

 evinced by monocular vision. Therefore it is particularly the sensation 

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

 positions either outside or inside from the centres of the two retina?, 

 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 



