Dr. Henneberg has read deeply in the extensive literature of spiritualism and occultism. He treats of table turning and spirit rapping, and what he calls psychographs, in which the subject, believed to be in a trance, writes to the dictation of the spirits of the dead. He mentions cases in which the mental derangement resulted from being hypnotised. Persons who practise these so-called communings with spirits have fallen into hysterical conditions. Some of these were previously disposed to neurosis, so that the tendency was merely aggravated. In other instances hysteria appeared as the result of these practices in persons previously healthy. Dr. Henneberg gives in detail the description of eight cases in which insanity was the result of concentration on spiritualistic experiments. He observes that it is the duty of the physician to warn persons disposed to nervous diseases against any dealings with mediums, magnetisers, and faith healers, who work on the imagination.