It may be pointed out here that in the case of a regular sitter who always marks his plates in the same way, as most do, it would not be at all difficult to forge his signature on a prepared plate and substitute this for one of the marked plates. _Group II._--_Other Methods_, conveniently classified as follows: (_a_).--Methods involving preparation of the studio: 1. An accomplice may be concealed behind the sitter, and be photographed with him; this is the simplest way of all, the sitter facing the camera, and, being told not to move during the exposure, is unaware that a “spirit face” is behind him, framed in an unsuspected opening in the background. Being behind the sitter, the face will be a little out of focus, and will appear rather blurred on the negative.[6] 2. It has been suggested that a mirror, or sheet of glass--on the principle of “Pepper’s Ghost”--may be introduced behind the sitter, producing the spirit by reflection of an accomplice hidden from the sitter. In practice this would be rather complicated and difficult to conceal; it would seem to have no advantage over the preceding method. 3. The extra is frequently sketched on the background--especially if this be a plain one--in some fluorescent substance, such as quinine sulphate. Such a sketch is invisible to the eye, but visible to the photographic plate. Many of Boursnell’s spirits appear to have been produced in this manner. (_b_).--Methods involving the camera and dark slides: 1. A trick slide may be employed, in which the shutter contains a positive transparency of the desired extra, held in such a manner that it can either be withdrawn with the shutter, or left in position in front of the plate when required; i.e., during the exposure, which will have to be somewhat longer than usual. 2. A similar transparency may be inserted in the camera, close to the plate, and between it and the lens, during the focussing operations. The black focussing-cloth makes an admirable screen for such manipulations, while the sitter is of necessity immobilised a few feet from the camera. It is easy to imagine how a transparency on a spring mount could be slipped into the camera under cover of the cloth in such a way as to press up against the plate when the shutter of the slide is drawn. 3. It is stated that a doubly refracting lens has been used, focussing onto the same plate both the sitter and an object concealed at one side of the studio. Such a contrivance may have been employed, but would certainly not be cheap to manufacture.