The duration of hypnotic sleep is as variable as that of its prototype in natural somnambulism. The patient usually wakes spontaneously, after a few minutes or hours. Sometimes, however, the period of insensibility is greatly prolonged. If it be desirable to awaken the subject of experiment, a simple reversal of the movements by which sleep was induced may suffice. The paroxysm may be terminated by almost any sudden and energetic appeal to the senses, like an electric shock, a sudden illumination of the eye with vivid light, or a sharp puff of air upon the face. The simplest phenomena connected with the hypnotic state are those transferences of cerebral perceptions which have been investigated by the Society for Psychical Research.[92] Certain sensitive persons, when blindfolded, are capable of reproducing with considerable accuracy visual images that have been impressed upon the mind of another. The sensitive subject is blindfolded and placed before a table with pencil and paper. Another person then goes out of the room, and gazes at some kind of drawing, geometrical figure, or other object selected without possibility of collusion with the subject of experiment. This person then returns to the room, and places his hand upon the head of the subject, at the same time fixing his attention upon the mental picture of the object. Presently the blindfolded subject takes the pencil and reproduces on paper a rough drawing of the object in question. In some cases it is found possible to effect this transfer of impressions without actual physical contact,--the agent merely standing behind the sensitive subject and concentrating his thought upon the selected object. Closely akin to this is the method of muscle-reading, popularly known as _mind-reading_. The sensitive is blindfolded, and then presses against his forehead the hand of the person by whom he is to be guided. Almost immediately a tremor pervades his muscles, and he yields all his movements to the guiding influence of the individual with whom he is in contact. If now an object be concealed in any place that is known to the agent, the concentration of that person’s attention upon the hiding place suffices to direct the “mind-reader,” who immediately drags his companion to the given locality.