52 LETTERS ON NATURAL MAGIC. 



frightful apparitions become nothing more than 

 our ordinary ideas, rendered more brilliant by 

 some accidental and temporary derangement of 

 the vital functions. Their very vividness, too, 

 which is their only characteristic, is capable of 

 explanation. I have already shown that the re- 

 tina is rendered more sensible to light by volun- 

 tary local pressure, as well as by the involun- 

 tary pressue of the blood-vessels behind it ; and 

 if, by looking at the sun, we impress upon the 

 retina a coloured image of that luminary, which 

 is seen even when the eye is shut, we may by 

 pressure alter the colour of that image, in conse- 

 quence of having increased the sensibility of that 

 part of the retina on which it is impressed. 

 Hence we may readily understand how the vivid- 

 ness of the mental pictures must be increased by 

 analogous causes. 



In the case both of Nicolai and Mrs. A. the 

 immediate cause of the spectres was a deranged 

 action of the stomach. When such a derange- 

 ment is induced by poison, or by substances 

 which act as poisons, the retina is peculiarly 

 affected, and the phenomena of vision are singu- 

 larly changed. Dr. Patouillet has described the 

 case of a family of nine persons who were all 

 driven mad by eating the root of the hyoscyamus 

 niger, or black henbane. One of them leapt into 

 a pond, another exclaimed that his neighbour 

 would lose a cow in a month, and a third vocife- 

 rated that the crown piece of sixty pence would 

 in a short time rise to five livres. On the fol- 

 lowing day they had all recovered their senses, but 

 recollected nothing of what had happened. On 

 the same day they all saw objects double, and, 

 what is still more remarkable, on the third day 



