"I don't follow you." "Two days ago I put a dummy letter addressed to Sydenham in his private lock-box at the Utinam. I had promised, you know, to send him on his mail if he would keep away from the club, and accordingly I had the key of the letter-box in my possession. Ten minutes later I went again to the box and it was empty--that is, you could see distinctly from one end of the box to the other, and it was absolutely bare." "A duplicate key, of course." "Not at all. It is only a stupid person who descends to crime--except as a last resort." "Well, then?" "Did you ever attend any of the exhibitions at the old Egyptian Hall? One of the favorite illusions was the trick cabinet in which the performer seated himself in full view of the spectators. The doors would be closed for an instant, and then, when reopened, the man had disappeared. The full interior of the cabinet was plainly visible; it stood on legs, which precluded the idea of a trap-door, and it was incontestably shown that egress from the back, top, or sides was impossible." "Yet the performer was gone?" "I said that the cabinet appeared to be empty--quite another thing." "Go on." "It was a simple arrangement of plate-glass mirrors fitting closely at the sides and backed by the distinctive pattern of wall-paper with which the rest of the cabinet was covered. Immediately that the doors were closed, the performer drew these false sides outward, so that they met the centre post of the doors at an acute angle. The true side walls were thereby exposed, and, of course, they were papered to correspond with the rest of the interior. Their reflection was doubled in the mirrors, making it appear to the observer that the whole cabinet was open to his vision. The truth was that he saw only half of it, the performer being concealed behind the mirrors. The only possible point at which the illusion could be detected was the angle where the mirrors joined, and this was masked by the centre post at which the double doors met. To conclude the trick, the doors were again closed, the performer swung the mirrors back into place, and, presto! he was back in the cabinet, smiling genially at the gaping crowd." "Then you think--"