On reaching the house, Boyd, an expert officer who had spent years in the investigation of crime, ascended with his subordinate to the drawing-room, while we remained on the ground floor to complete our search, the sergeant being stationed inside the hall. Our further investigations were not very fruitful. The fact that dinner was laid for three indicated that a third person had been present, or was expected. The room did not differ from any other, except that it was perhaps better furnished than one would have expected in such a house, for although in a first-class and rather expensive neighbourhood the row of houses had declined in popularity of late years, and was now inhabited mostly by the lodging-house fraternity. In moving about the room, however, my coat caught the plate laid for the person who was to occupy the head of the table, and it was nearly swept off. I saved it, however, but beneath was revealed a plain white card which, until that moment, had been concealed. Patterson caught sight of it at the same moment, and taking it in my hand I examined it, finding that it was a plain visiting card of lady's size, one side being blank, and other bearing a roughly-drawn circle in ink. There was nothing else. "That's certainly curious," my companion remarked, looking over my shoulder. "Yes," I said, lifting a second plate to see what was there concealed, and finding another card, in all appearances similar, plain, but bearing across its reverse a single straight line drawn with a pen. "By Jove!" observed Patterson, lifting the other plate, and finding a third card, "this is certainly very strange." He turned the card over, but it was blank on both sides. "I wonder what game is this, or whether these have any connexion with the crime?" I exclaimed, holding all three of the cards in my hand, turning them over and examining them carefully beneath the light. "By the ink they have the appearance of having been prepared long ago. See!" I added, holding one of them towards him, "the corners of this one are slightly turned up and soiled. It has been carried in some one's pocket, and is not a fresh card." Again Patterson took it and examined it. It was the one with the line drawn across it. The others were quite clean, as if just taken fresh from a packet. "There's some mystery about these," he said reflectively, as though speaking to himself. "If we could but solve it we should likewise solve the problem of the crime, depend upon it." "No doubt," I assented. "Each of them have some meaning, occult but extraordinary. They were turned face downwards so that the accidental removal of the plate would not reveal the device upon them." "The devices are simple enough, but undoubtedly they have some hidden meaning," my friend said. "They were evidently concealed there, and the three persons, unsuspecting, were to discover them when the first plates were removed," I suggested. He placed them together on the table, saying--