Mr. A. Claudet on Binocular Vision, 



549 



The nature of the decomposition of gun-cotton when exploded 

 under different conditions is now under investigation by me ; and 

 the results arrived at will, I trust, be communicated before long to 

 the Royal Society. 



April 1 1 . — Lieut. -General Sabine, President, in the Chair. 

 The following communication was read : — 



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

 F.R.S. 



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

 demonstrated 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 " Vic- 

 toria," 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 suffi- 

 cient to complete the word on each surface, which blank spaces are 

 in fact to appear filled up during the experiment hy an artificial 

 means to he explained. 



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

 view) . 



Fig. 1. 



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. 



a- v d c r. o i I 



Now by means of two strings fixed on the two sides A, B 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 communicated to the card, and while it is revolving 

 both surfaces are alternately seen in quick succession, and the percep- 

 tion of the two is so simultaneous 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 



Phil Mag. S. 4. No. 226. Suppl. Vol. 33. 2 



