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into it. There can be no doubt of the mental disorder of persons who suffer in this way, but it must not be supposed that hallucinations of sight do not occur to persons who are free from mental disorder. I cannot help thinking that they furnish the explanation of the firm belief in ghosts and apparitions which has prevailed among all nations and in all times. A belief so universal must have some deep foundation in the facts of Nature or in the constitution of man. One may freely admit that persons have seen apparitions and have heard voices which they thought to be supernatural; but, inasmuch as seeing is one thing, and the interpretation thereof quite another thing, it may be right to conclude that they were nothing more than hallucinations, and that the reason why no ghosts are seen now, when people pass through churchyards on dark nights, as our forefathers saw them, is that ghosts are not believed in nowadays, while we have gained a knowledge of the nature of hallucinations, and of the frequency of their occurrence, which our forefathers had not.

One does not fail to notice, when proper attention is given to the subject, a fact which is full of meaning, viz., that the apparitions which have been seen at different ages were in harmony with the dominant ideas or beliefs of the age. It is not probable that any one could be found at the present day to affirm that he had seen an old woman riding through the air on a broomstick to a witches' meeting, because the belief in witchcraft is happily wellnigh extinct; but two or three hundred years ago, when it would have been thought something like blasphemy to doubt the being and doings of witches, persons of character and veracity might have been found to avouch it solemnly. In like manner, apparitions of Satan were not very uncommon in the middle ages to persons who, like Luther, were in earnest spiritual conflict with him; but there is no instance on record, so far as I know, of such an apparition having ever been seen by an ancient Greek or Roman. The Satan of the middle ages who gave Luther so much trouble had not then been invented. Spirits, ghosts, then, and all apparitions of the same kind, I was prepared to have pronounced unhesitatingly to have been hallucinations, which would be found on examination to reflect pretty fairly the prevailing ideas of the time concerning the supernatural; but it occurred to me that it might be prudent, before doing that, to consult the article on apparitions in the latest edition of the "Encyclopædia Britannica," lest perchance I should be outrunning current authority; and I have there discovered, to my no small surprise, that it is still an open question whether invisible inhabitants of the unknown world did not take human or other shapes and become visible to men. The writer of the article plainly inclines to the opinion that they do, and that there is more in the matter than science has yet dreamed of. So also think the spiritualists.

I now go on to consider the mode of production of hallucinations. At the first blush there might seem to be a great gap between such