BUT NOT IMPLYING DISTINCT DEPARTURE FROM HEALTH a. A kind of hallucination to which perhaps every normal per son is liable is that known technically as "recurrent sensation." This kind is experienced only when some sense-organ has been continuously or repeatedly subjected to some one kind of impres sion or stimulation for a considerable period; e.g., the microsco pist, after examining for some hours one particular kind of ob ject or structure, may suddenly perceive the object faithfully re produced in form and colour, and lying, as it were, upon any surface to which his gaze is directed. Perhaps the commonest experience of this type is the recurrence of the sensations of movement at intervals in the period following a sea voyage or long railway journey.
b. A considerable proportion of healthy sane persons can in duce hallucinations of vision by gazing fixedly at a polished sur face or into some dark translucent mass; or of hearing, by apply ing a large shell or similar object to the ear. These methods of inducing hallucinations, especially the former, have long been practised in many countries as modes of divination, • various ob jects being used, e.g., a drop of ink in the palm of the hand, or a polished finger-nail. The object now most commonly used is a polished sphere of clear glass or crystal (see CRYSTAL-GAZING). Hence such hallucinations go by the name of crystal visions. The crystal vision often appears as a picture of some distant or unknown scene lying, as it were, in the crystal; and in the picture figures may come and go, and move to and fro, in a perfectly natural manner. In other cases, written or printed words or sen tences appear. The percipient, seer or scryer, commonly seems to be in a fully waking state as he observes the objects thus pre sented. He is usually able to describe and discuss the appearances, successively discriminating details by attentive observation, just as when observing an objective scene; and he usually has no power of controlling them, and no sense of having produced them by his own activity. In some cases these visions have brought back to the mind of the scryer facts or incidents which he could not voluntarily recollect. In other cases they are asserted by credible witnesses to have given to the scryer information, about events distant in time or place, that had not come to his knowledge by normal means. These cases have been claimed as evidence of telepathic communication or even of clairvoyance. But at present the number of well-attested cases of this sort is too small to jus tify acceptance of this conclusion by those who have only second hand knowledge of them.
c. Prolonged deprivation of food predisposes to hallucinations, and it would seem that, under this condition, a large proportion of otherwise healthy persons become liable to them, especially to auditory hallucinations.
d. Certain drugs, notably opium, Indian hemp and mescal pre dispose to hallucinations, each tending to produce a peculiar type. Thus Indian hemp and mescal, especially the latter, produce in many cases visual hallucinations in the form of a brilliant play of colours, sometimes a mere succession of patches of brilliant colour, sometimes in architectural or other definite spatial ar rangement.
e. The states of transition from sleep to waking, and from waking to sleep, seem to be peculiarly favourable to the appear ance of hallucinations. The recurrent sensations mentioned above are especially prone to appear at such times, and a considerable proportion of the sporadic hallucinations of persons in good health are reported to have been experienced under these conditions. The name "hypnagogic" hallucinations, first applied by Alfred Maury, is commonly given to those experienced in these transition states.
f. The presentations, predominantly visual, that constitute the principal content of most dreams, are generally described as hallucinatory, but the propriety of so classing them is very ques tionable. The present writer is confident that his own dream presentations lack the sensory vividness which is the essential mark of the percept, whether normal or hallucinatory, and which is the principal, though not the only, character in which it differs from the representation or memory-image. It is true that the dream-presentation, like the percept, differs from the representa tive imagery of waking life in that it is relatively independent of volition ; but that seems to be merely because the will is in abeyance or very ineffective during sleep. The wide currency of the doctrine that classes dream-images with hallucinations seems to be due to this independence of volitional control, and to the fact that during sleep the representative imagery appears without that rich setting of undiscriminated or marginal sensation which always accompanies waking imagery, and which by con trast accentuates for introspective reflection the lack of sensory vividness of such imagery.
g. Many of the subjects who pass into the deeper stages of hypnosis (see HYPNOTISM) show themselves, while in that con dition, extremely liable to hallucination, perceiving whatever ob ject is suggested to them as present, and failing to perceive any object of which it is asserted by the operator that it is no lodger present. The reality of these positive and negative hallucinations of the hypnotized subject has been recently questioned, it being maintained that the subject merely gives verbal assent to the sug gestions of the operator. But that the hypnotized subject does really experience hallucinations seems to be proved by the cases in which it is possible to make the hallucination, positive or neg ative, persist for some time after the termination of hypnosis, and by the fact that in some of these cases the subject, who in the post-hypnotic state seems in every other respect normal and wide awake, may find it difficult to distinguish between the halluci natory and real objects. Further proof is afforded by experiments such as those by which Alfred Binet showed that a visual hallu cination may behave for its percipient in many respects like a real object, e.g., that it may appear reflected in a mirror, displaced by a prism and coloured when a coloured glass is placed before the patient's eyes. It was by means of experiments of this kind that Binet showed that hypnotic hallucinations may approximate to the type of the illusion, i.e., that some real object affecting the sense-organ (in the case of a visual hallucination some detail of the surface upon which it is projected) may provide a nucleus of peripherally excited sensation around which the false percept is built up. An object playing a part of this sort in the genesis of an hallucination is known as a "point de repere." It has been maintained that all hallucinations involve some such point de repere or objective nucleus; but there are good reasons for re jecting this view.
h. In states of ecstasy, or intense emotional concentration of attention upon some one ideal object, the object contemplated seems at times to take on sensory vividness, and so to acquire the character of an hallucination. In these cases the state of mind of the subject is probably similar in many respects to that of the deeply hypnotized subject and these two classes of hallucination may be regarded as very closely allied.