Apparition, a spectral illusion, by which imaginary objects are presented to the senses with such vividness that they are believed to be real. This form of illusion, the result of some abnormal state of the brain, concerning which medical science has given thus far only incomplete information, has been the cause of much superstition. The apparitions seen in actual delirium, or by those obviously insane, do not of course fall within the scope of this article; and the well authenticated instances in which apparitions have been seen by men of ordinarily clear intellect, and apparently in their customary good health, are so mingled with impostures and exaggerations that it is difficult to make them the ground of scientific investigation. But there are some cases where men of the highest intellectual power have had this cerebral affection, yet have retained enough acuteness of observation to investigate their own disease, and describe the apparitions coolly and accurately, though knowing them to be illusory.

The most noteworthy of these cases is that of Nicolai, an eminent publisher in Berlin, who in 1791 was for some months constantly subject to spectral illusions, which presented to him the figures of friends, unknown persons, and singular animals, which accompanied him everywhere, went through all the movements belonging to their real prototypes, and even spoke to him. Conscious of their character, he observed them so accurately as to be able to write a scientific paper upon them for the philosophical society of Berlin. He was ultimately cured by blood-letting. Many similar instances are recorded in the volumes referred to at the close of this article. - Some well authenticated accounts of apparitions appearing to persons a short time before death do not in the present state of medical inquiry admit of so satisfactory an explanation. That both the apparitions actually seen and those in which the superstitious believe should most frequently represent the forms of dead friends, is conceded to be natural; for the brains of those who see or fancy they see them are generally excited by grief or filled with morbid fears of death.

Yet these causes, and the natural tendencies of superstitious minds and low states of knowledge, gave rise to the popular belief in ghosts. - See Dr. John Ferriar's "Essay toward a Theory of Apparitions" (London, 1813); Dr. Samuel Hibberfs " Sketches of the Philosophy of Apparitions" (Edinburgh, 1824); Sir Walter Scott's "Letters on Demonology and Witchcraft" (Edinburgh, 1830); Mrs. Crowe's "Night Side of Nature " (London, 1848); Jung Stilling's Geisterkunde, translated into English under the title "Pneumatology ' (New York, 1851); Dr. Brierre de Boismont's "Hallucinations, or the Rational Theory of Apparitions," etc. (English translation, Philadelphia. 1853); Robert Dale Owen's "Footfalls on the Boundary of Another World" (Philadelphia, 1860), and his "Debatable Land " (New York, 1872). See also, in this work, Demonology, Spiritualism, and Witchcraft.