]867.] 



Mr. A. Claudet on Binocular Vision. 



425 



The means by which this illusion is produced has been called the *' thau- 

 matrope/' from two Greek words meaning "wonder" and "turn." It 

 is difficult to trace the history of this discovery ; but it is certain that it 

 has been the result of a very old, simple, and well-known experiment. 



From time immemorial schoolboys have amused themselves by holding a 

 coin between two pins and making it revolve rapidly by blowing upon it, 

 when to their surprise the coin showed the head mixed with the device on the 

 other side. I have been told that as Sir John Herschel was one day making 

 this experiment to amuse his children, in the presence of the late Dr. Paris, 

 this gentleman was struck with the idea that if, instead of a coin, a white 

 card was employed on each side of which one part of a design was properly 

 arranged, the two might complete the subject during the revolution. Ac- 

 cordingly he made the experiment, which succeeded perfectly well. If the 

 story is true, certainly Dr. Paris may be regarded as the inventor of the 

 thaumatrope, which he has so well and so fully elucidated in his very in- 

 teresting and instructive work entitled ' Philosophy in Sport made Science 

 in earnest.' 



This philosophical toy may be employed to show another effect of the 

 persistence of the retinal image. If complementary colours are fixed on 

 the two opposite sides of the card, they will become superposed during the 

 revolution, and white light will be the result. By the same means, other 

 curious effects of the mixture of various colours might be tried. 



All these experiments present no difference, whether they are made look- 

 ing with the two eyes or only with a single eye : the effect is the same in 

 both cases. Therefore the illusion is equally monocular and binocular. 



But I was not a little surprised to find that the thaumatrope is capable 

 of producing another phenomenon, elucidating very forcibly the principles 

 by which binocular vision is the only real and effective means of showing 

 the distances of objects, which are determined by the degree of the angle 

 of convergence of the optic axes and by one of its corollaries, the sensation 

 of double images for all the points which are not exactly on the plane of 

 vitiion. 



The thaumatrope is capable of showing that binocular vision can detect 

 to a degree hardly conceivable the most minute difference in the distances 

 of objects, such as the distance between the planes of the two surfaces of 

 a card, which distance is nothing more than the thickness of the card. 

 Therefore, supposing that the thickness of the card is -y^of an inch and the 

 distance from the eyes 15 in, there is not a difference greater than the jyoiS 

 part of the whole distance from the eyes to the two planes of the card ; and 

 still the difference of the degree of convergence for two planes so near each 

 other is sufficient to excite the action of binocular vision, and by it to enable 

 us to detect 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 pheno- 



