Experimental Telepathy and Clairvoyance.—Experi ments in thought-transference have been conducted for many years past in most civilized countries, and the conditions of the experiments have been diverse. Sometimes the percipient has been in a normal waking condition, sometimes in hypnosis ; some times in the same room or house as the agent, sometimes at a dis tance of many miles, or even in a different country. The subjeit matter has also varied greatly ; attempts have been made to transfer impressions of numbers, simple or complex diagrams, the suit and value of playing cards, landscapes or pictures, inci dents from books and imaginary scenes. Records of various experiments on these lines will be found in S.P.R. Proceedings passim, and in books such as N. W. Thomas's Thought Trans ference (1905), Tischner's Telepatliie and Hellsehen, Warcol lier's La Telepathic.
In experiments with numbers or a pack of cards it is possible to make a statistical computation as to how far (if at all) the successes exceed those which can be assigned to chance; the mathematics are not quite as simple as is sometimes supposed (see Proc., vol. xxxiv., p. 185 sq.). In experiments with dia grams, scenes and incidents, no such computation is, of course, possible.
Where agent and percipient are in the same room, the pos sibility of hyperaesthesia (i.e., an unusual acuity of the normal senses of sight, hearing, etc.) must be taken into account. This is the explanation which Prof. Gilbert Murray is disposed to accept for the remarkable successes obtained by him in receiving im pressions of incidents from books or imaginary scenes (see S.P.R. Proceedings, vols. xxix. and xxxiv.). Prof. Murray was not, in fact, in the same room as the agents, and for this and other reasons it is difficult to accept hyperaesthesia as the ex planation. It is greatly to be regretted that Prof. Murray has not continued the experiments under conditions which would leave no reasonable doubt as to the operative cause.
Another factor, which does not always receive sufficient atten tion, is that of fatigue or boredom on the part of the percipient. From recent experiments in clairvoyance by Miss Jephson in England (S.P.R. Proc., vol. xxxviii.) and Estabrooks in America (Boston S.P.R. Bulletin 5) it is clear that there is a well-marked "fatigue curve," that is to say, there is a tendency for percipients to score most of their successes in the early stages of an experi ment, while in the later stages the results may fall definitely below the line of chance probability, apparently as the result of some inhibition due to boredom or confusion.
For experiments where the agent and percipient are widely separated see S.P.R. Proceedings, vol. xxi. Experiments are now in progress between a French group, organized by Warcollier, and an English group organized by S. G. Soal.
Reference has already been made to Miss Jephson's recent experiments in clairvoyance, but so far much less has been ac complished with clairvoyance than with telepathy. The usual
form of experiment is to set the clairvoyant to "guess" the suit and value of cards taken from a shuffled pack held face down wards, or the content of writings enclosed in envelopes externally identical in appearance ; if the experiment is properly carried out nobody has normal knowledge of the correct answer, and telep athy is therefore excluded. Where the clairvoyant is not a per son whose bona fides is absolutely above suspicion, the same precautions against deception and the same alertness on the ob server's part are necessary as in investigating "physical" phe nomena. The reading of the contents of sealed letters, in par ticular, is a favourite trick of dishonest mediums. For some successful experiments with sealed envelopes, in which the in vestigators were fully on their guard, see the Report of the War saw International Congress (1923), p. 201.
The Modus Operandi in Telepathy and Clairvoyance.— While taking the spontaneous and the experimental evidence to gether, there is a strong case of accepting telepathy, and a fair case for accepting clairvoyance, there are still many obscure problems as to the manner in which these two faculties function. It may, for instance, be asked : Are the faculties common to all humanity, or confined to particular individuals? How far does the exercise of them depend on conscious effort? In what form are impressions conveyed, as words, as ideas, as visual images? Are they conveyed by means of any physical "waves" or "effluence"? To some of these questions it is possible to suggest a tentative answer. Ideas may be telepathically transmitted without con scious deliberate effort on the part of either agent or percipient, transmission taking place between their subliminal minds direct.
In some cases the agent justifies his name by deliberately willing to transmit impressions; in others it is possible that it is the per cipient's subliminal mind which goes to look for impressions, so to speak, rather than passively receives them. The fact that the process takes place subliminally makes it impossible to estimate how frequently it occurs. It has even been suggested that every subliminal is constantly transmitting impressions to, and receiving them from, every other subliminal, and that from time to time something happens which, as regards a par ticular percipient, raises the height of the unperceived, ever flow ing stream above the level of consciousness. It is hard to imagine any kind of evidence which could either prove or disprove such an hypothesis, which seems to have been invented simply as a weapon to be used against spiritists, when all others fail (see p. 671, "Evidence for Survival").