(2) Hyperæsthesia.—But, after all, it is rarely necessary to take special precautions against fraud, for there are dangers to be guarded against of a more subtle kind. There are various, and as yet imperfectly known methods of communication by which indications may be unconsciously given and as unconsciously received. Thus, to take the last instance, it is pretty certain that cornea-reading does not always imply fraud, and that hints may be gained in all good faith from any reflecting surface in the neighbourhood of the experimenter; or the movements of lips, larynx, and even hands and limbs may betray the secret to eye or ear. We know little of the limits of our sensory powers even in normal life; and we do know that in certain subconscious states—automatic, hypnotic, somnambulic—these limits may be greatly exceeded, and that indications so subtle as frequently to escape the vigilance of trained observers may be seized and interpreted by the hypnotic or automatic subject. It is clear, therefore, that results which it is possible to attribute to deliberate fraud stand almost necessarily self-condemned. For if the precautions taken by the investigators left such an explanation open, much more were those precautions insufficient to guard against the subtler modes of communication referred to. It is not the friend whom we know whose eyes must be closed and his ears muffled, but the "Mr. Hyde," whose lurking presence in each of us we are only now beginning to suspect.
There is a case recorded by M. Bergson,[8] in which a hypnotised boy is said to have been able to state correctly the number of the page in a book held by the observer, by reading the corneal image of the figures. The actual figures were three millimetres high, and their corneal image is calculated by M. Bergson to have been O.1 mm., or about ½50 of an inch in height! In some other experiments conducted by M. Bergson with the same subject the acuteness of vision is said to have exceeded even this limit. In another case, recorded by Dr. Sauvaire,[9] a hypnotised subject was able to recognise the King of Clubs, face downwards, in two different packs of cards. In the first of these cases the results, which could not have been attained by the senses under normal conditions, must apparently be attributed to hyperæsthesia. Instances, especially of auditory hyperæsthesia, are of course quite familiar to those who have studied the phenomena of hypnotism. In Dr. Sauvaire's case, however, the power of distinguishing the cards by touch may have been the result of practice. Mrs. Verrall records (Proceedings Soc. Psych. Research, vol. viii. p. 480) that she acquired such a power by means of "a longish series of experiments"; and Mr. Hudson, in Idle Days in Patagonia, tells of a gambler who by careful training had developed the same faculty in a very high degree.
It seems probable in the cases described by M. Bergson and Dr. Sauvaire, and possible also in the case of Mr. D.'s subject, that there was no intentional deception, and that the hypnotised person was not himself aware of the means by which his knowledge was attained.[10] The same remark probably applies to the following case, in which, though the conditions of vision were certainly unusual, it seems not clear whether the degree of success attained should be attributed to abnormal sensibility of the eyes, or to the facility acquired by long practice. In a series of experiments at which the writer assisted, in 1884, an illiterate youth named Dick was hypnotised, a penny was placed over each eye, and the eyes and surrounding features were elaborately bandaged with strips of sticking-plaster; a handkerchief being bound over all. Under these conditions Dick named correctly objects held in front of him, even at a considerable distance, a little above the level of his eyes. Normal vision appeared to be impossible. Mr. R. Hodgson, however, repeated the experiment upon himself, and found after several trials that he also could see objects, though fitfully and imperfectly, under the same conditions, the channel of vision being a small chink in the sticking-plaster on the line where it was fastened to the brow.
(3) Muscle-reading.—From this last case we may pass to the illustrations of "thought-reading" given by professional conjurers and others, where it seems clear that the skill exhibited in the interpretation of unconscious movements and gestures is due rather to long practice and careful observation than to any abnormal extension of faculty. It hardly needs saying that experiments in which contact is permitted between the agent and percipient can rarely be regarded as having evidential value. It has been demonstrated again and again that with the fullest intention of keeping the secret to themselves, most "agents" in such circumstances are practically certain to betray it to the professional thought-reader by unconscious movements of some kind. Indeed, it is difficult to place any limit to the degree of susceptibility to slight muscular impressions which may be attained. A careful experimenter has assured the writer that when acting as percipient in some experiments with diagrams the slight movements of the agent's hand resting upon her head gave her in one case a clue to the figure thought of. And Mr. Stuart Cumberland has exhibited feats still more marvellous before kings and commoners. Nor is it necessary, as already said, for successful muscle-reading that there should be actual contact in all cases. The eye or the ear can sometimes follow movements of the lips or other parts of the body. But though we can look for little evidence from experiments conducted with contact, or under conditions which allow of interpretation by gesture, etc., and their repetition in this connection can rarely be expected to serve any useful purpose, it seems worth pointing out that, if telepathy is a fact, we should expect to find it operating not merely where, from the conditions of the experiment, it must be presumed to be the sole source of communication, but also as an auxiliary to other more familiar modes of expression. It seems not improbable, therefore, that some of the more startling successes of the professional "thought-reader" and some of the results obtained in the "willing game" may be due to this cause.
(4) Thought-forms.—There remains one other source of error to be guarded against. An image—whether of an object, diagram, or name—which is chosen by the agent may be correctly described by the percipient simply because their minds are set to move in the same direction. It must be remembered that, however unexpected and spontaneous they may appear, ideas do not come by chance, but have their origin mostly in the previous experience of the thinker. Persons living constantly in the same physical and intellectual environment are apt to present a close similarity in their ideas. It would not even be prima facie evidence of thought-transference, for instance, if husband and wife, asked to think of a town or of an acquaintance, should select the same name. And investigation has shown that our thoughts move in grooves which are determined for us by causes more deep-seated and more general than the accident of particular circumstances. Thus it is found that individuals will show a preference for certain figures or certain numbers over others; and that the preference for some geometrical figures tends to be tolerably constant. The American Society for Psychical Research[11] made some interesting observations on this point in 1888. Blank cards were issued to a large number of persons, with the request that the recipients would draw on the card "ten diagrams." 501 cards were returned, and the diagrams inscribed on them were carefully tabulated. It was found that of the 501 persons no less than 209 drew circles, 174 squares, 160 equilateral triangles and crosses, while three only drew wheels, two candlesticks, and one each a corkscrew, a ball, and a knife. It was found that the simpler geometrical figures[12] occurred not only most frequently but as a general rule early in each series of ten. It follows, therefore, that in an experiment the success of the percipient in reproducing a circle, a square, or a triangle raises a much fainter presumption of thought-transference than if the object reproduced had been a corkscrew or a pine-apple. But so much was perhaps obvious even without a detailed investigation. From a similar analysis of the guesses made, it can be shown that some percipients have decided preferences amongst the simple numerals. And in the same way it seems probable that others have a preference for particular cards. An important illustration of the working of the "number-habit" has been brought forward by Professor