SIGHT. 575 



engravings of Sommerring, are distinctly seen greatly magnified 

 and projected apparently on a plane before the eye. The image 

 continues only so long as the flame is in motion : when the flame 

 becomes stationary, it immediately dissolves into fragments and 

 soon after disappears. Prof. Wheatstone finds that the sus- 

 ceptibility of seeing it depends upon the size of the pupil : the 

 image readily appears to those whose pupils are large, while, 

 on the contrary, the experiment rarely succeeds with a person 

 whose pupil is small. I can see the blood-vessels of my own 

 retina and the blood circulating through them, just as in a frog's 

 web under the microscope, by merely closing my eyes and ex- 

 cluding the light by placing my hand before them. The phe- 

 nomenon occurs in this way more readily at one time than at 

 another: and some persons say they never perceive it. ,*, 



A view of the retina, with the ramification of its central 

 artery. In the centre is seen its central hole, surrounded 

 by a circle. To one side of this is the bulb of the optic 

 nerve. 



Purkinje has attempted no explanation of this phenomenon, but 

 contented himself with simply stating the fact. Prof. Wheat- 

 stone considers that it is a shadow, resulting from the obstruction 

 of light by the blood-vessels spread over the retina* The difficulty, 

 he observes, is not to account for the appearance of the image, 

 but to explain why this shadow is not always visible. He adduces 

 a number of facts observed by Pictet, Sir D. Brewster, and others, 

 which tend to prove that an object, either more or less luminous 

 than the ground on which it is placed, becomes invisible when 

 continuously presented to the same point of the retina, the ra- 

 pidity of its disappearance being greater as the difference of 

 luminous intensity between the object and the ground is less: 

 but, by continually shifting the place of the object on the retina, 

 or by making it act intermittingly on the same point, the object 

 may be rendered permanently visible. To apply this explanation to 

 Purkinje's experiment, Prof. Wheatstone observes that, whenever 

 the flame of the candle changes its place, the shadows of the 

 vessels fall on different parts of the retina ; as is evident from 



Q Q. 



