Not till ten years later did a photographer named Hudson succeed, with the aid of a medium, in producing spirit pictures. The _modus operandi_ appeared simple. The sitter was posed before the camera, and the picture was subsequently developed, when besides the sitter's own image there appeared another figure or figures usually draped, with the features blurred or only partly distinguishable. Usually these figures were recognised unhesitatingly by the sitters as portraits of deceased relatives or friends. Afterwards the practice of spirit photography received a rude shock. They were examined carefully by professional photographers, and some of them were found to bear clear marks of double exposure, the background in each case being visible through the dress of the sitter--a fatal defect in spirit photography. Moreover it was found that in some cases the medium had dressed up to play the rôle of spirit. Whereupon several of those who had professed to recognise the "ghosts" now hastened to repudiate their recognition. But spirit photography was not to be quashed so easily. The experiments went on, and faces and figures appeared on the developed plate which seem to have considerably baffled the experts. Sir William Crookes now resolved to put the matter to a test by attempting to obtain a photograph of "Katie," the famous "control" of Miss Cook, the medium. The young lady gave a series of sittings in May 1874 at Sir William's house for the purpose. These sittings took place by electric light, no fewer than five cameras being simultaneously at work. The medium lay down on the floor behind a curtain, her face muffled in a shawl. When the materialisation was complete "Katie" would appear in the full light in front of the curtain: "I frequently," writes Sir William Crookes, "drew the curtain on one side when Katie was standing near; and it was a common thing for the seven or eight of us in the laboratory to see Miss Cook and Katie at the same time, under the full blaze of the electric light. We did not on these occasions actually see the face of the medium, because of the shawl, but we saw her hands and feet; we saw her move uneasily under the influence of the intense light, and we heard her moan occasionally. I have one photograph of the two together, but Katie is seated in front of Miss Cook's head."