Whether the elimination of all other possible causes is indeed complete, or whether, as in Dr. Bastian's case, there may yet lurk in these experiments some hitherto unsuspected source of error, the reader will have the opportunity of judging for himself. To assist him in forming a judgment some of the main disturbing causes will be briefly indicated. (1) _Fraud._--In nearly all the experiments referred to in this book the agent was himself concerned in the inquiry as a matter of scientific interest. But it necessarily happens on occasion that neither agent nor percipient are by education and position absolutely removed from suspicion of trickery in a matter where trickery might to imperfectly educated persons appear almost venial. If any such cases have been admitted, it is because the precautions taken appear to us to have been adequate. At the same time, the investigators of the Society for Psychical Research have come across some instances of fraud in cases where they had grounds for assuming good faith, and it may be useful, therefore, to illustrate some of the less obvious methods of acquiring intelligence fraudulently. The conditions of the experiment should of course, as far as possible, preclude, even where there is no ground for suspecting fraud, communication between the percipient and the agent, or any one else knowing the idea which it is sought to transfer. In the autumn of 1888 some experiments were conducted with a person named D., whose antecedents afforded, it was thought, justification for the belief that the claims which he put forward were genuine. D. acted as agent, the percipient being a subject of his own, a young woman called Miss N., who was apparently in a light hypnotic sleep during the experiment. It was soon discovered that the results were obtained by means of a code formed from a combination of Miss N.'s breathing with slight noises--a cough or the creak of a boot--made by D. himself. I have seen a somewhat similar code employed in Prince's Hall, Piccadilly, where the conjurer stood in the middle of the hall with a coin or other object in his hand, a description of which he communicated to his confederate on the platform by means of a series of breathings, deep enough visibly to move his dress-coat up and down on the surface of his white collar, punctuated by slight movements of head or hand. The novel feature in the first case, however, was that the percipient herself furnished the groundwork of the code, the punctuation alone being given by the conjurer. A still more elaborate form of collusion is described at length by Bonjean.[7] In this case the subject, a young woman named Lully, appears to have read the words to be conveyed after the fashion of a deaf mute, by the motion of the lips of the showman. Lully was apparently in a hypnotic trance, with the eyes fast closed. Another form of fraud, since it does not require the aid of a confederate, is perhaps worthy of note. Some years