DUEATION OF LUMINOUS IMPRESSIONS. 27 



may even exhibit at the same instant the two 

 opposite sides of the same object, or two pictures 

 painted on the opposite sides of a piece of 

 card. It was found by a French philosopher, M. 

 ' D' Arcet, that the impression of light continued 

 on the retina about the eighth part of a second 

 after the luminous body was withdrawn, and upon 

 this principle Dr. Paris has constructed the pretty 

 little instrument called the T/taumatropa, or the 

 Wonder-turner. It consists of a number of cir- 

 cular pieces of card, about two or three inches 

 broad, which may be twirled round with great 

 velocity by the application of the fore-finger and 

 thumb of each hand to pieces of silk string at- 

 tached to opposite points of their circumference. 

 On each side of the circular piece of card is 

 painted part of a picture, or a part of a figure, in 

 such a manner that the two parts would form a 

 group or a whole figure, if we could see both 

 sides at once. Harlequin, for example, is painted 

 on one side, and Columbine on the other, so that 

 by twirling round the card the two are seen at 

 the same time in their usual mode of combination. 

 The body of a Turk is drawn on one side, and 

 his head on the reverse, and by the rotation of 

 the card the head is replaced upon his shoulders. 

 The principle of this illusion may be extended to 

 many other contrivances. Part of a sentence 

 may be written on one side of a card, and the 

 rest on the reverse. Particular letters may be 

 given on one side, and others upon the other, or 

 even halves or parts of each letter may be put 

 upon each side, or all these contrivances may be 

 combined, so that the sentiment which they ex- 

 press can be understood only when all the scattered 

 parts are united by the revolution of the card. 



