5. An extra may be painted on the inner surface of the dark-slide shutter, in some radio-active chemical. The shutter usually only clears the surface of the sensitised emulsion by a fraction of a millimetre, and a fairly distinct extra will be produced if the plate is kept in the slide for a sufficient length of time--depending, of course, upon the amount of radio-active substance used.
1. In the days of the wet-plate process, when plates were cleaned and used a second time with fresh emulsion, it would sometimes happen that the original photograph would re-develop on top of the second, very careful chemical cleaning of the plate being necessary to prevent this. Mumler’s first spirit photograph was probably produced in this way, and the knowledge was turned to good account by several of the earlier spirit photographers. Some of the unexpected results obtained by amateurs may be attributable to this cause, because a certain number of used plates are returned to plate manufacturers, who clean off the emulsion and use the glass again. The cleansing may sometimes be imperfect, and in these cases the original image may appear on development.
2. Faces may be sketched in chemicals on small pieces of card, or even on the medium’s fingers. On opportunity arising in the dark-room, the medium holds or steadies the plate for an instant, bringing the chemical pictures into contact with the plate. Or he may so manoeuvre it that the plate is laid face down on a prepared surface of the dark-room work-bench, probably while it is being marked[7]; upon development of the plate extras will duly appear. The most refined version of this method consists in the preparation of small rubber stamps in which the chemicals are smeared. These can easily be palmed and dabbed for a moment on the plate in a manner which appears quite unsuspicious. A number of active chemicals will produce this effect, but the medium must be careful to know whether the substance he is using will accelerate or retard development in the affected part; for cases have occurred in which a positive extra has been produced on the negative plate, giving a negative spirit on the finished print!
3. Mr. Bush, in his recent pamphlet, “Spirit Photography Exposed,” describes a piece of apparatus made out of an empty blacking-tin containing a small electric bulb, one side of the tin being replaced by a positive transparency of the desired extra. This, he alleges, is used by Hope, the Crewe spirit photographer, the transparency being pressed against the plate and the light switched on for a second. If carefully faced with black velvet round the transparency, this device should be quite useful; but it must be remembered that an escaping ray of white light would at once catch the eye in the dark-room. Skilful palming and manipulation should make it quite possible for an extra to be printed on the plate in this way, if the medium can cover it with his hand for a moment or two. All Hope’s results are certainly not produced in this way, however, as is implied by Mr. Bush.