Yet, on all other occasions, save that of their immediate presence, I found no difficulty in assigning their existence to a diseased state of the bodily organs, and a corresponding sympathy of the mind, rendering it capable of receiving and reflecting the false, fantastic, and unnatural images presented to it.
(One of our most celebrated medical writers considers spectral illusion a disease, in which false perceptions take place in some of the senses; thus, when the excitement of motion is produced in a particular organ, that organ does not vibrate with the impression made upon it, but communicates it to another part on which a similar impression was formerly made.

Nicolai states that he made his illusion a source of philosophical amusement.

The spectres which haunted him came in the day time as well as the night, and frequently when he was surrounded by his friends; the ideal images mingling with the real ones, and visible only to himself.

Bernard Barton, the celebrated Quaker poet, describes an illusion of this nature in a manner peculiarly striking:-- "I only knew thee as thou wert, A being not of earth! "I marvelled much they could not see Thou comest from above And often to myself I said, 'How can they thus approach the dead ?' "But though all these, with fondness warm, Said welcome o'er and o'er, Still that expressive shade or form Was silent, as before! And yet its stillness never brought To them one hesitating thought." "I recollected that the mode of exorcism which was successfully adopted by Nicolai of Berlin, when haunted by similar fantasies, was a resort to the simple process of blood-letting.

I accordingly made trial of it, but without the desired effect.