"He didn't," Quarles answered shortly. "It is obvious that a man who possessed such stones as were found in that packet at the bank would certainly not make such a mistake; yet he was apparently playing with his treasure when he met his death. My theory had three points, you see. First, the sapphire was the sole object of the robbery; secondly, the thieves had substituted an exact duplicate for the real stone; thirdly, the stone must have some special fascination for Mr. Ratcliffe, or he would have put it in the bank for safety as he had done with others." "An interesting theory, I admit, but----" "Wait, Mr. Wigan. I have said something about my methods. I began to look for facts to support my theory. You remember the cook-housekeeper?" "Perfectly." "She spoke of her uncle's sensitiveness to noises; she had on one or two occasions surprised him in a listening attitude. That gave me a clew. What was he listening for? Mr. Ratcliffe had only given way to this listening attitude recently; in fact, only since his return from his last voyage. It would seem that since his return his mental balance had become unstable. There was some constant irritation in his brain which brought fear, and in his dead eyes there was terror. My theory was complete; I had only to fit the facts into it. I suppose, Mr. Wigan, you have found out all about the people living on either side of Ratcliffe's house?" "Both are families above suspicion," I answered. "I also tried Ossery Road, the gardens of which run down to those on that side of Blenheim Square. The house immediately behind No. 12 is occupied by a doctor." "I know. I called upon him recently to put some scientific point to him," said Quarles with a smile. "I came to the conclusion that he could give me no information about Mr. Ratcliffe. Rather curiously, he did not like Mr. Ratcliffe." "So I discovered," I answered, and I was conscious of resenting the professor's active interference in the case. There is no telling what damage an amateur may do. "His dislike was a solid fact," said Quarles. "I congratulate you on not being put on a false scent by it. Many detectives would have been. The gardens end on to each other--a doctor, a knowledge of subtle poisons--oh, there were materials for an excellent case ready to hand." "We are getting away from the point, professor," I said, somewhat tartly.