426 Mr. A. Claudet on Binocular Vision. [April 11, 



menon I have discovered, and which I am about to describe and endeavour 

 to explain. 



If the thickness of the card is A B, fig. 3, and if the two ends of each 



Fig. 3. 



string, passing through the holes C and D in the card, are brought together 

 and turned between the thumb and finger, the card will whirl exactly on 

 its axis, and during: the revolution the two surfaces A and B will be at the 

 same distance from the eyes. 



But if the two strings are drawn so that one of their knots is as in fig. 4, 



Fig. 4. 



the surface B will revolve round the plane of the surface A corresponding 

 with the axis of the string, and, during the revolution, every time that it is 

 made visible to the eyes it will appear as if it were nearer than the sur- 

 face A. 



By reversing the position of the knots, as in fig. 5, instead of the surface 



Fig. 5. 



B revolving round the plane of A, it will be A that will revolve round the 

 plane of B. 



These three different positions of the strings will produce three diflPerent 

 effects. 



In the position of fig. 3 the effect will be normal ; that is to say, the two 

 surfaces coming alternately at the same distance, we shall see the whole 

 word as if the letters were on the same surface. 



In the position of figs. 4 and 5 we have a very strange illusion. One 

 half of the letters composing the word will appear before or behind the 

 other half, according to the surface upon which they are written and the 

 position of the knots upon that or the other surface. 



In fig. 4 the letters written on the surface B will appear before the letters 

 on the surface A ; and in fig. 5 the letters on the surface A will appear 

 before the letters on the surface B. 



The cause of the anomaly resulting from the two different experiments 

 is entirely and positively due to a sensation of binocular vision ; and we may 



