There is, however, one important factor lacking in the conjurer's performance to illustrate completely the psychology of deception; it is that the mental attitude of the observer is too definite. He knows that he is being deceived by skill and adroitness, and rather enjoys it the more, the more he is deceived. He has nothing at stake; his mind rests easy without any detailed or complete explanation of how it was done. Quite different must have been the feeling of the spectator before the necromancer of old, in whose performance was seen the evidence of secret powers that could at a moment's notice be turned against any one to take away good luck, to bring on disease, or even to transform one into a beast. When magic spells and wonder-working potions were believed in, what we would now speak of as a trick was surrounded with a halo of awe and mystery by the sympathetic attitude of the spectators. The most complete parallel to this in modern times is presented by the physical phenomena of Spiritualism; and so many of the manifestations presented by performing mediums in evidence of Spiritualism have been exposed and proven to be conjuring tricks, that it is no longer an assumption to consider them in this connection. Spiritualistic phenomena present a perfect mine for illustrations of the psychology of deception, and it is these that I shall consider as the final topic in this cursory view. The first general principle to be borne in mind is that the medium performs to spectators _in doubt_ as to the interpretation to be placed upon what they see, or more or less prepared or determined to see in everything the evidence of the supernatural. This mental attitude on the part of the spectators is worth more to the medium than any single factor in the performance. The difference between such a presentation and one addressed to persons cognizant of the conjuring element in the performance and interested in its detection, cannot be exaggerated. It is this that makes all the difference between the séance swarming with miracles, any one of which completely revolutionizes the principles of science, and the tedious dreariness of a blank sitting varied only by childish utterances and amateurish sleight-of-hand. Careful observers often report that the very same phenomena that were utterly beyond suspicion in the eyes of believers are to unprejudiced eyes so apparent "that there was really no need of any elaborate method of investigation"; close observation was all that was required, and Mr. Davey, who conducted an admirable investigation of the reliability of accounts of sleight-of-hand performances, has experimentally shown that of equally good observers, the one who is informed of the general _modus operandi_ by which such a phenomenon as "slate-writing" is produced will make much less of a marvel of it than one who is left in doubt in this regard.