576 SIGHT. 



the motion of the figure, which, while the eye remains at rest, is 

 always in a contrary direction to that of the flame. Hence the 

 shadow, being thus made to change its place on the retina, re- 

 mains, according to the law above stated, permanently visible ; 

 but, instantly the flame is at rest, the shadow also becomes sta- 

 tionary and consequently disappears. 



Sir D. Brewster offers a different explanation. He considers that 

 the light is propagated from the luminous image of the candle to 

 other parts of the retina, and that, though the retina, in contact 

 with the blood-vessels, is sensible to direct light, it is insensible 

 to propagated light, and therefore the blood-vessels are delineated 

 in obscure lines. This explanation does not agree with the fact 

 that the vessels continue to be seen only while the flame is in 

 motion. 



Prof. Wheatstone has described several original and instructive 

 variations of the experiment, for which we must refer to his own 

 account. 111 



Duration of luminous impressions on the retina. The effect of 

 light upon the eye continues for some time after the light itself 

 has ceased to act. If a red-hot coal be rapidly whirled round so 

 as to return to the precise point from which it started, it will 

 produce a perfect ring of light, although the coal can be at one 

 point only of the circle at the same instant. Chevalier D'Arcy 

 was the first who made this experiment the subject of philosophical 

 consideration : he ascertained that the impression on the retina 

 remained about the eighth of a second. Prof. Wheatstone has 

 devised a very instructive experiment to illustrate this property 

 of vision. A narrow slit is made in a disc from the centre to the 

 circumference : when this disc is stationary and held before a 

 window blind or a strongly illuminated picture, a very limited 

 portion only of the object is seen through the linear aperture ; 

 but, on causing it to revolve rapidly on its centre, the different 

 portions of the picture, which are seen through the disc in its 

 successive positions, remaining on the eye, the entire object is 

 visible at the same time and the solid disc appears perfectly 

 transparent. 



The stationary appearances of moving wheels as observed by 

 Drs. Roget and Faraday, Dr. Paris's thaumatrope, Professor 



m Contributions to the Physiology of Vision. Journal of the Royal Institution, 

 Nos. 1 . and 3. 



