Description of the Thaumatrope. 87 



strated, that the choroid coat is essential to vision, we may 

 ascribe the loss of red light in certain eyes to the retina itself 

 having a blue tint. If this should be the case, the light 

 which falls upon the choroid coat will be deprived of its red 

 rays, by the absorptive power of the blue retina, and conse- 

 quently the impression conveyed back to the retina, by the 

 choroid coat, will not contain that of red light. 



No VI. Description of the Thaumatrope. 



The Thaumatrope, (or the wonder-turner, from (Jai>/x« a 

 wonder, and Tgivo to turn,) a very ingenious philosophical toy, 

 invented, we believe, by Dr Paris, is founded on the well- 

 known optical principle, that an impression upon the retina 

 continues for about the eighth part of a second after the ob- 

 ject which produced it is withdrawn. The luminous rings 

 formed by the whirling of a burning stick in the dark are 

 well known, and Homer has availed himself of the same prin- 

 ciple in his description of the lengthened shadow of the flying 

 javelin. 



The Thaumatrope consists of a number of circular pieces 

 of card, about two and a half inches in diameter, which may 

 be twirled round with great velocity by the application of the 

 fingers to pieces of silk string attached to two opposite points 

 of their circumference. On each side of the card is painted a 

 part of a picture, so that if we could see both sides at once, 

 the two parts of the picture would form a whole picture. For 

 example, in Plate I. Fig. 8., we have shown two sides of a 

 card, on one of which is a cage, and on the other a bird. If 

 wc now take hold of each of the silk strings A and B, between 

 the fore-finger and thumb of each hand, and put it into a 

 twirling motion, the bird and the cage will appear to the eye 

 at the same moment, in consequence of the impression of each 

 continuing on the retina for a short space of time. The fol- 

 lowing are some of the other devices on the cards of the thau- 

 matrope : 



A rose-tree, with a garden pot on the reverse. 



A horse, with a man on the reverse. 



