But we are obliged, for the sake of clearness of evidence, to set aside, when dealing with experimentation, all these mixed emotional cases, and to start from telepathic communications intentionally planned to be so trivial, so devoid of associations or emotions, that it shall be impossible to refer them to any common memory or sympathy; to anything save a direct transmission of idea, or impulse, or sensation, or image, from one to another mind. The reader who has studied the evidence originally set forth in Chapters II. and III. of _Phantasms of the Living_ will, I trust, carry away a pretty clear notion of what can at present actually be done in the way of experimental transferences of small definite ideas or pictures from one or more persons--the "agent" or "agents"--to one or more persons--the "percipient" or "percipients."[106] In these experiments actual _contact_ has been forbidden, to avoid the risk of unconscious indications by pressure. It is at present still doubtful how far close proximity really operates in aid of telepathy, or how far its advantage is a mere effect of self-suggestion--on the part either of agent or of percipient. Some few pairs of experimenters have obtained results of just the same type at distances of half a mile or more.[107] Similarly, in the case of induction of hypnotic trance, Dr. Gibert attained at the distance of nearly a mile results which are usually supposed to require close and actual presence. [See Appendix V. C.] We must clearly realise that in telepathic experiment we encounter just the same difficulty which makes our results in hypnotic therapeutics so unpredictable and irregular. We do not know how to get our suggestions to _take hold_ of the subliminal self. They are liable to fail for two main reasons. Either they somehow never _reach_ the subliminal centres which we wish to affect, or they find those centres preoccupied with some self-suggestion hostile to our behest. This source of uncertainty can only be removed by a far greater number of experiments than have yet been made--experiments repeated until we have oftener struck upon the happy veins which make up for an immense amount of sterile exploration. Meantime we must record, but can hardly interpret. Yet there is one provisional interpretation of telepathic experiment which must be noticed thus early in our discussion, because, if true, it may conceivably connect our groping work with more advanced departments of science, while, if seen to be inadequate, it may bid us turn our inquiry in some other direction. I refer to the suggestion that telepathy is propagated by "brain-waves"; or, as Sir W. Crookes has more exactly expressed it, by ether-waves of even smaller amplitude and greater frequency than those which carry the X rays. These waves are conceived as passing from one brain to another, and arousing in the _second_ brain an excitation or image similar to the excitation or image from which they start in the _first_.