“While we are here we may as well clear up the mystery of the luminous footprints, for I see the removal of this square of ceiling has already disclosed a part of the mechanism. You will observe here, a glass tube, above which there are two lead plates. The top plate is movable, and is connected with an electro-magnetic device for moving it. In the bottom plate is cut in miniature, the shape of a human footprint. The glass tube is what is known as a Crookes Tube, and the rays which emanate from it when an electric connection is established are known as X-rays. Although these rays are, in themselves, invisible, some of them have the property of making certain substances phosphorescent. The rays which have this property can be cut off by a lead screen of the correct thickness. One of the substances which can be rendered luminous is sulfid of zinc, and is probably the one used, although I have not yet had an opportunity to verify this. The substance, whatever it may be, has probably been ground into exceedingly minute particles and rubbed into the rug above our heads. A luminous footprint can thus be made to appear on the rug by the simple expedient of turning on the current in the Crookes Tube and sliding back the upper plate in such a manner that the toe prints will first be visible, then the ball of the foot, and finally the heel. I’m sure that if we remove more squares of plasterboard we will find a row of these contrivances about two feet apart, leading to a point beneath the center of the room, where two of them will be found side by side. For the present, however, we will go upstairs to continue our investigation in other directions.” When we were once more in the living room, the doctor asked for a stepladder, and Riggs was sent to bring one. When he brought it, the doctor placed it in the center of the room and climbed up to where the central lighting fixture projected from the wall. “In this fixture,” he said, “are concealed one of the sources of the icy breath, and also the source of the ghastly and foul-smelling spectre which rose from the center of the floor on two succeeding evenings. You will observe that the entire fixture, central hemisphere and surrounding collar, appears to be made from frosted glass. The central hemisphere from which the light emanates is glass, but the surrounding collar is of metal covered with a white substance. That white substance is common frost.” So saying, he scraped off a quantity of the frost and handed it down to us for our inspection. “Please take special notice of the designs on this collar,” he said, “for they are particularly well suited for the purposes for which our friend Hegel intended them--a series of circles, each about an inch from the other, reaching entirely around the collar. I will now do by force what the builder of this device previously did by mechanical means, controlled by radio.”