DURATION OF IMPRESSION OF LIGHT. 295 



which never fails to strike the fly into the sea, where it soon becomes 

 its prey. 1 Hommel a Dutch governor put some of these fish into a 

 tub of water, and pinned a fly on a stick within their reach. He daily 

 saw the fish shoot at the fly, and with such dexterity, that they never 

 failed to hit the mark. 2 Pallas describes the Sisena jaculatrix as 

 securing flies by a similar contrivance. 3 



If the light, before reaching the eye, passes through bodies of a len- 

 ticular shape, it undergoes modifications, which have given occasion to 

 the formation of useful instruments devised for modifying the sphere 

 of vision. If the lens be double convex, the body, seem through it, 

 appears larger than it is, from the illusion, so often referred to, that 

 we always refer the object in the direction of the line that impinges 

 upon the retina. The object, consequently, appears to be greatly aug- 

 mented. (See Fig. 83.) For the same reasons an object seems smaller 

 to the eye at A, Fig. 80, when viewed through a double concave lens. 

 Again, if the light, before reaching the cornea, be made to pass through 

 a diaphanous body, which is itself coloured, and consequently allows 

 only the rays of its own colour to traverse it, the object is not seen of 

 its proper colour, but of that of the transparent body. 



An impression of light continues to affect the retina for some time 

 after the impression has ceased, certainly for the sixth part of a second. 4 

 If, therefore, a live coal be whirled round, six or seven times in a second, 

 it will seem to be a continuous circle of fire. It is owing to this cir- 

 cumstance, that meteors seem to form a line of light as in the case 

 of the falling star; and that the same impression is conveyed by a sky- 

 rocket in its course through the air. We have an elucidation of the 

 same fact in the instrument or toy called, by Dr. Paris, thaumatrope 

 which consists of a circle, cut out of a card, and having two silken 

 strings attached to opposite points of its diameter : by twisting these 

 with the finger and thumb the card may be twirled round with consider- 

 able velocity. If we make on one side a black stripe as in the marginal 

 figure 132, and on the other side one at right angles to it, Fig. 133, and 

 cause the card to revolve rapidly, we shall see a cross. And if on one side 



Fig. 132. Fig. 133. 



Thaumatrope. 



of the card a chariot is drawn and on the other a charioteer, and the card 

 be twirled round six or seven times in a second, the charioteer will be 

 seen in the chariot, the duration of the impressions on the retina being 



i Fleming's Philos. of Zoology, i. 195. 2 Philos. Trans., liv. 89. 



3 Philos. Trans., Ivi. 186; also, Mr. Sharon Turner's Sacred History of the World, Amer. 

 edit., i. 205, New York, 1832. 



4 D'Arcy, Memoires de 1'Academie des Sciences, p. 439, Paris, 1765; and Plateau, Annales 

 de Chimie, &c., vol. Iviii. p. 401. 



