[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. Fearful, from the representations of my physicians, and from some of my own sensations, that the almost daily recurrence of my visions might ultimately lead to insanity, I came to the resolution of reducing my daily allowance of opium; and, confining myself, with the most rigid pertinacity, to a quantity not exceeding one third of what I had formerly taken, I became speedily sensible of a most essential change in my condition. A state of comparative health, mental and physical with calmer sleep and a more natural exercise of the organs of vision, succeeded. I have made many attempts at a further reduction, but have been uniformly unsuccessful, owing to the extreme and almost unendurable agony occasioned thereby.