2. If the sitter brings the right plates he will show the packet to the medium before entering the dark-room to make sure that they are all right. The medium takes the packet into his hand for a moment--turning to the light to read the label--and passes them back with the remark that they are the right kind--which now they certainly are, for the sitter’s original packet is in the medium’s breast-pocket. 3. The sitter may perhaps autograph or otherwise mark his packet before coming to the medium, in order to prevent any such substitution. In this case the medium will wait until the wrapper is torn off in the dark-room, when he may be able to handle the box for a moment on some pretext,[4] and the dim light makes the substitution easier than before, particularly as it occurs during the first minute or so in the reduced light before the sitter’s eyes have become accustomed to it. If these methods are employed, the medium usually finds it necessary previously to mark the plate or plates in the box that have the latent extras, in such a way that he may be sure of not getting the spirit inverted: a slight scratch on one edge will suffice for this. (_b_).--Methods involving substitution of the faked plate only, after removal from the original packet: 1. With an unwary sitter this may be done in the dark-room. The sitter usually marks the plates; while he is marking one, the medium may be able to exchange his prepared plate for one of those not yet marked. 2. A trick dark-slide may be used, having a secret partition, and already containing the faked plate.[5] If the sitter is content to mark the plate after it is placed in the slide, he may easily be caused to mark the prepared plate instead of his own. 3. If the plates are not marked, it will be a simple matter to substitute, during the focussing operations, a duplicate slide containing a faked plate. 4. Little accidents are apt to happen in the unaccustomed light of the red lamp; while the sitter is groping on the floor for a wrapper he has dropped, or while his attention is in some other way diverted for a moment, the exchange is made. I am aware that many will ridicule the idea of such a simple trick being played upon an intelligent observer; but any conjuror, whose business it is to do this kind of thing, knows that it is remarkably easy. 5. Sometimes the first photographs taken are blanks, the sitter then returns to the dark-room and loads up some fresh plates out of the packet. It may not occur to him that an accomplice of the medium has had access to the dark-room in the meantime, and when he gives this account of the _séance_ a few days later he will probably have entirely forgotten that the plates were not all loaded at once. Substitution can, of course, be effected in many other ways; every medium probably has his favourite method which he chiefly practises.