In this domain, the psychologist and the "psychical researcher" proceed most amicably; and yet their purposes and points of view lead them frequently to part company, although it may be only for a brief _au revoir_. When the "psychical researcher" leaves the main highway to track a possible "telepathic" hypnotic subject, or one who, while hypnotized, is sensitive to the magnetic current, or who experiences the characteristic effects of drugs applied in sealed vials to the back of the neck, or who falls into the hypnotic condition when handling a "magnetized" doll,--the psychologist is apt to decline the invitation to join in the pursuit. I should advise him, however, to go along for the sake of the excellent illustrations thus obtainable of the effects of unconscious suggestion. From the time of the first serious investigation of these phenomena up to the present, unconscious suggestion has been one of the most potent influences for the production of alleged marvels and pseudo-phenomena. All the series of experiments brought forward at irregular intervals during the past century to establish supernormal sensibilities have depended for their apparent success (apart from trickery) upon unconscious suggestion of the operators, combined with the shrewd assimilation of the desired or expected result on the part of the subjects. The transposition of the senses discovered by Pétetin (1787), the hypnotized subjects who in Braid's day (1850) proved the location of the phrenological organs by the appropriateness of their actions when certain parts of the head were pressed, the sensitiveness to magnets and hermetically sealed drugs brought forward by Reichenbach (1845), and by Bourru and Burot (1885), and Dr. Luys's (1890) absurd trifling with puppets, and probably, too, Charcot's sharp differentiation of distinct hypnotic conditions (1882),--all furnish illustrations of the subtle possibilities of unconscious suggestion. Besides adorning an interesting psychological tale, they point a moral to the intending investigator, and open his eyes to the extreme caution necessary to exclude this source of error, and to realize the ever-present possibility that, in spite of the sterilizing apparatus and the other equipments of modern research, the germs of this insidious form of delusion may have been unwittingly introduced.