Watching him closely, I was aware that he became more irritable as he proceeded. One small cabinet, which might possess a secret hiding-place, he broke with the chisel, and I noticed that whenever a drawer was locked his scrutiny of the contents was more careful. He evidently expected that the man he was robbing would value the thing he was looking for, and would be likely to hide it securely. He had worked round half the room when he suddenly stopped, and, with a quick movement, took up the revolver. I had not heard a sound in the house, but he had. There was no sign of doubt in his attitude, which was of a most uncompromising character. He did not make any movement to switch off the light, he did not attempt to conceal himself. He just raised his arm and pointed the revolver toward the door, on a level at which the bullet would strike the head of a man of average height. The handle was turned, and the door began to open. The next five seconds were full of happenings. For just a fraction of time I realized that the burglar meant to shoot the intruder without a word of warning, and for a moment I seemed unable to utter a sound. Then I shouted: "Back for your life!" Immediately there was a sharp report. Quarles had fired from behind the Chesterfield, and the burglar's arm dropped like a dead thing to his side, his revolver falling to the floor. "Quickly, Wigan!" Quarles cried. I had dashed aside the curtain, and I threw myself upon the burglar just in time to prevent his picking up his weapon with his left hand. He struggled fiercely, and I was glad of Tresman's help in securing him, although the doctor had come perilously near to losing his life by his unexpected intrusion. But for Christopher Quarles he would have been a dead man. We called in the police, and, when our prisoner had been conveyed to the station, the professor and I went back to Chelsea. "Do you know what he was looking for, Wigan?" Quarles asked. "Something in bone or ivory." "Bone," answered Quarles. "Thank heaven that fool Tresman didn't come sooner! We might have missed much that was interesting. You noted how keen he was with every piece of bone he could find, how irritable he was growing. The counters, Wigan, they were the clew. But I did not understand their significance at first." "I do not understand the case now," I confessed, "except that we have caught a mad burglar."