Mr. A. Claudet on Binocular Vision. 



551 



tect that infinitesimal difference in their distances. But that such 

 an effect of binocular vision could possibly be displayed while looking 

 at two planes so nearly intermixed as the surfaces of a card revolving 

 upon its axis with such a wonderful velocity is the very extraordinary 

 phenomenon 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 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 sur- 

 faces A and B will be at the same distance from the eyes. 



But if the two strings are drawn t so that one of their knots is as in 

 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 surface A. 



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



Fig. 5. 



Surface B revolving round the plane of A, it will be A that will re- 

 volve round the plane of B. 



These three different positions of the strings will produce three 

 different 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. 



202 



