"I went to his room," the detective hurried on, "and found a lot of his clothes and his stationery and his toilet articles marked with the same cipher. I knew my man had made a big mistake--the sort of mistake every criminal makes no matter how clever he is--and I had him. But that isn't, by any means, all. Don't look so distressed, Mr. Graham. There isn't the slightest chance for him. You see I repaired the lock, and, as soon as it was day, closed the room and went outside to look for signs. Since nightfall no one had come legitimately through the court except Doctor Groom and myself. Our footprints were all right--making a straight line along the path to the front door. In the soft earth by the fountain I found another and a smaller print, made by a very neat shoe, sir, and I said to myself: 'There is almost certainly the footprint of the murderer.' "There were plenty of others coming across the grass. He'd evidently avoided the path. And there was one directly under the open window where the body lies. It's still there. Perhaps you can see it. No matter. That's the last one I found. The prints ceased there. There wasn't a one going back, and I was fair up a stump. Then I saw a little undefined sign of pressure on the grass, and I got an idea. 'Suppose,' I says, 'my man took his shoes off and went around in his stockinged feet!' I couldn't understand, though, why he hadn't thought of that before. I went back to Robert Blackburn's room and got one of his shoes, and ran into a snag again. The sole of the shoe was a trifle larger than the footprints. Every one of his shoes I tried was the same way. I argued that the handkerchief was enough, but I wanted this other evidence. I simply had to clear up these queer footprints.