GENERAL VIEWS. 133 



very vividness too, which is their only characteristic, is 

 capable of explanation. I have already shown that the 

 retina is rendered more sensible to light by voluntary 

 local pressure, as well as by the involuntary pressure 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 con- 

 sequence of having increased the sensibility of that part 

 of the retina on which it is impressed. Hence we may 

 readily understand how the vividness 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 derangement is induced by poison, 

 or by substances which act as poisons, the retina is 

 peculiarly affected, and the phenomena of vision singularly 

 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 

 vociferated that the crown piece of sixty pence would in 

 a short time rise to five livres. On the following 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 every object appeared to them as red as scarlet. 

 Now this red light was probably nothing more than the 

 red phosphorescence produced by the pressure of the 

 blood-vessels on the retina, and analogous to the masses of 

 blue, green, yellow, and red light, which have been already 

 mentioned as produced by a similar pressure in headaches, 

 arising from a disordered state of the digestive organs. 



Were wo to analyse the various phenomena of spectral 



