424 



Mr. A. Claudet on Binocular Vision. 



[April 11, 



April \\,\m7. 

 - Lieut.-General SABINE, President, in the Chair. 

 The following communications were read : — 



I. *'A new fact relating to Binocular Vision." By A. Claudet, 

 F.R.S. Received March 20, 1867. 



The persistence of the impression made by light on the retina is demon- 

 strated by many experiments ; but one of the most convincing, which is also 

 very easy to try, is that which is known under the name of the thaumatrope. 



Let us write the letters composing a word of eight letters, say " Victoria," 

 on the two sides of a small card, in such a manner that one surface shall 

 contain the 1st, 3rd, 5th, and 7th letters, and the other surface the 2nd, 

 4th, 6th, and 8th, with a space between them sufficient to complete the 

 word on each surface, which blank spaces are in fact to appear filled up 

 during the experiment by an artificial means to be explained. 



Fig. 1 shows the arrangement on the two sides of the card (section view). 



Fig. 1. 



V C O I 

 I T R A 



Fig. 2 shows the plan of the card. The white letters are those written 

 on one surface, and the dotted lines those written on the other. 



Fig. 2. 



Now by means of two strings fixed on the two sides A, E the card may 

 be made to revolve on its axis by turning the string between the thumb 

 and finger of each hand. By this means a very rapid motion may be com- 

 municated to the card, and while it is revolving both surfaces are alter- 

 nately seen in quick succession, and the perception of the two is so simulta- 

 neous that the two sets of letters appear as one, and the whole word is read 

 as distinctly as if it were written on one surface only. 



This is easily explained. It is known that the persisting action of light 

 on the retina has a duration of about one-eighth of a second ; so that if 

 the card makes at least eight revolutions in a second (it may make consi- 

 derably more), before one impression has vanished another produces its 

 eifect on the next part of the retina, in such a way that they are intermixed 

 and simultaneously visible, producing an uninterrupted sensation. 



