There are certain phenomena which lend themselves badly to observation: this is particularly the case with those which require obscurity and arrangements of a nature likely to hinder or interfere with the best control which can be exercised, that of the eyesight. In my opinion the phenomenon has no demonstrative value whenever it occurs out of sight, as is the case with slate-writing, when the slate is held under the table. Neither has it any great signification when it requires sustained observation in order to control it. Errors are easy, for abstraction almost inevitably follows, if it does not accompany, sustained attention. Hodgson, in ‘The Possibilities of Mal-Observation and Lapse of Memory from a Practical Point of View’ (_Proceedings_, iv. 381) gives examples of this, but his paper only points out facts well known to those who are familiar with human testimony. In order to observe with a minimum chance of error, the phenomenon we intend to study should be simple, and repeated often enough to prevent the attention from becoming weary from waiting. From this point of view, the production of raps and telekinetic movements with the aid of the experimental manœuvres I have described, permit, by specifying the moment when the phenomenon is going to occur, of bringing the whole attention to bear upon the examination of the conditions under which the phenomenon is obtained. Raps and movements without contact appear to me to lend themselves admirably to observation; with these phenomena, by operating as I have indicated, experimentation is almost possible; but a veritable medium must be sought for in the first instance. Now this is what my colleagues of the Society for Psychical Research did, but they did so under conditions which were far from satisfactory. Mrs. Sidgwick, a woman of brilliant intellect, has given an account of the attempts made by herself, her husband, and friends to obtain psychical phenomena. They went to Eglinton and Slade for slate-writing, to the Misses Wood and Fairlamb and a Mr. Haxby for materialisations. The first two gave phenomena which were suspicious, not to say worse; as for Haxby, he frauded shamefacedly. Mrs. Sidgwick’s account is demonstrative on this point, and it is enough to read it to be convinced that no shrewd observer could be taken in.