550 Royal Society : — 



may make considerably more), before one impression has vanished 

 another produces its effect on the next part of the retina, in such a 

 way that they are intermixed and simultaneously visible, producing 

 an uninterrupted sensation. 



The means by which this illusion is produced has been called the 

 " thaumatrope," 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 em- 

 ployed on each side of which one part of a design was properly ar- 

 ranged, the two might complete the subject during the revolution. 

 Accordingly he made the experiment, which succeeded perfectly well. 

 If the story is true, certainly Dr. Paris may be regarded as the in- 

 ventor of the thaumatrope, which he has so well and so fully elu- 

 cidated in his very interesting and instructive work entitled \ Philo- 

 sophy 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 su- 

 perposed 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 

 looking 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 vision. 



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 -^ of an inch and the distance from the eyes 15 in., there is 

 not a difference greater than the y^Vo 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 de- 



