26 LETTERS ON NATURAL MAGIC. 



rays of the sun, which, through a small opening 

 in the clouds, was throwing its light only upon 

 that part of the landscape. The black figure was 

 projected against a white cloud, and the white 

 horse shone with particular brilliancy by its con- 

 trast with the dark soil against which it was seen. 

 A person interested in the arrival of such a 

 stranger had been for some time following his 

 movements with intense anxiety, but, upon his 

 disappearance behind a wood, was surprised to 

 observe the spectre of the mounted stranger in 

 the form of a white rider upon a black steed, and 

 this spectre was seen for some time in the sky, 

 or upon any pale ground to which the eye was 

 directed. Such an occurrence, especially if ac- 

 companied 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 dis- 

 tance 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 consist 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 whirl- 

 ing 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 



