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 are 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 we to analyse the various phenomena of spectral illusions, we should discover many circumstances favourable to these views. In those seen by Nicolai, the individual figures were always somewhat paler than natural objects. They sometimes grew more and more indistinct, and became perfectly white; and, to use his own words, “he could always distinguish with the greatest precision phantasms from phenomena.” Nicolai sometimes saw the spectres when his eyes were shut, and sometimes they were thus made to disappear,--effects perfectly identical with those which arise from the impressions of very luminous objects. Sometimes the figures vanished entirely, and at other times only pieces of them disappeared, exactly conformable to what takes place with objects seen by indirect vision, which most of those figures must necessarily have been.