It is important to note that in cases of this kind there is no discoverable _point de repère_, at least in the sense in which the phrase is understood by its authors; and the nature of the effect produced--a moving figure, apparently occupying a position in solid space--makes it very difficult to suppose that the hallucination is attached to any external object, which must necessarily be fixed. But the whole discussion about the necessity of external excitation or of _points de repère_ seems beside the mark in such cases as these. For there can be no question that what in the first instance excites the hallucination is not a present sensation, but a memory. Whether for the full development of a sensory hallucination some external stimulus to the sense-organ is necessary is here a question of quite minor importance. The really interesting fact in its bearing on the question of telepathic hallucination is that some hallucinations are shown to be centrally, not peripherally, initiated. It should be further remarked that Zillah's astonishment at seeing the figure is typical, since in the case of post-hypnotic hallucinations in general neither the injunction to see the figure, nor indeed any other incident of his trance life, is remembered by the percipient in the normal state; and he is therefore entirely ignorant of the chain of events which led up to the hallucination, and can only by inquiry and reflection ascertain that the apparition which he has seen is of his own manufacture.