DURATION OF LUMINOUS IMPRESSIONS. Ill 



accompanied with a suitable combination of events, might, 

 even in modern times, have formed a chapter in the history 

 of the marvellous. 



It is a curious circumstance, that when the image of 

 an object is impressed upon the retina only for a few 

 moments, the picture which is left is exactly of the same 

 colour with the object. If we look, for example, at a 

 window at some distance from the eye, and then transfer 

 the eye quickly to the wall, we shall see it distinctly but 

 momentarily with light panes and dark bars ; but in a 

 space of time incalculably short, this picture is succeeded 

 by the spectral impression of the window, which will con- 

 sist of black panes and white bars. The similar spectrum, 

 or that of the same colour as the object, is finely seen in the 

 experiment of forming luminous circles by whirling round 

 a burning stick, in which case the circles are always red. 



In virtue of this property of the eye an object may be 

 seen in many places at once ; and we 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 withdrawn, and upon this principle Dr.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 con- 

 structed the pretty little instrument, called the Thauma- 

 trope, or the Wonder-turner. It consists of a number of 

 circular pieces of card about two or three inches broad, 

 which may be twirled round with great velocity by the 

 application of the forefinger and thumb of each hand 

 to pieces of silk string attached 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 



