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in which the shutter contains a positive transparency of the desired extra, held in such a manner that it can either be withdrawn with the shutter, or left in position in front of the plate when required; i.e., during the exposure, which will have to be somewhat longer than usual.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.
4. A simpler method of obtaining the same result is to have a pinhole in the bellows of the camera; a brightly illuminated object at the side and rather in front of the camera will then throw an image on the plate. A considerable exposure will be needed to give a fair extra; but this will present no difficulties, as the pinhole will be open all the time the plate is in position, and not merely during the few seconds that the lens is uncapped for the photograph of the sitter.