Let us first of all consider the cases in which persons have caused themselves to vanish and reappear at will. This power of becoming visible and invisible to others is not limited to mythical times, but may be reproduced today by artificial means. If a sensitive subject be hypnotized (and there is some analogy to the hypnotic pass in the fact that the fairy invariably waved her wand before the eyes of the onlooker), hallucinations of various types may be induced. Thus, our subject may be persuaded to see, for instance, a dog walking across the carpet, whereas there is no dog there. He may be persuaded that there is a stream in front of him flowing through the drawing-room, and that it is necessary for him, in order to prevent his feet from becoming wet, to take off his shoes and socks, and turn up his trousers. Hypnotic suggestion will perform this, and it may be said that suggestion alone, even when the subject is not in the hypnotic state, may be employed to produce many of these hallucinatory pictures. On the contrary, it is possible to suggest to our subject that such and such an object is gradually diminishing in size, and finally that it disappears altogether. He sees and describes this diminution, and finally looks in vain for the object which, he asserts, has vanished, but which, as a matter of fact, is perfectly visible to all others not under the influence of the suggestion. We frequently suffer from these "negative hallucinations," as they are called, in our ordinary daily life. We cannot find an object which is perfectly visible--resting in the very centre of the area over which we are searching diligently. Suddenly we discover it; it seems incredible to us that we have not seen it before; it seems to have sprung into being as though placed there by some invisible hand. Nevertheless it had remained throughout in the one position, and the only remarkable factor was our inability to see it. Such cases are well known to psychologists (the power of suggestion in inducing both positive and negative hallucinations), and this--both in the normal and the hypnotic state--is well recognized. Now it is only necessary for us to extend our conception somewhat in order to see the scientific truths contained in many fairy stories, in which one of the characters--hero, fairy, or what not--becomes visible and invisible at will. It is only necessary for us to conceive that some degree of mental influence had been brought to bear upon the minds of the onlookers, and that suggestion had been skilfully employed, in order to account for many of these stories. I know of a case in which the operator made his subject, who remained practically in a normal state throughout, see him floating about the room--whisking over chairs and tables, as though the law of gravity had no further influence upon him!