"It's an impertinent observation, Doctor Marcy, but you say that _no one_ saw Mr. Graeme alive after your departure from the library at ten o'clock?" "Oh, I have my alibi straight enough," smiled the doctor. "Miss Trevor happened to be passing through the hall as I left the room. I stopped and spoke to her, made some jesting remark about Graeme's being good for a thousand years, more or less. At that same moment he came to the library door and waved his hand to us both; then he turned back, and we heard the click of the spring-latch. I believe that he usually set the catch when he wanted to make sure of not being disturbed. "Now we come to hypothesis No. 2, the possible assailant. The door leading into the hall was locked. There are no roof openings. The windows of stained glass in leaded frames are immovable; otherwise there would be danger of the valuable glass being broken or knocked out through an accidental jar. But for purposes of ventilation there is inserted in each section a pridella. Ah, you don't understand--come over here." Doctor Marcy conducted me across the room to the window on the right of the fireplace, the one depicting the return of the spies from the land of Canaan. "You will notice," he said, "that there are three panels in the window, each carrying a part of the general picture. Then, in the lower part of the central panel, there is a small subsidiary scene; in this particular case it represents a field of waving wheat in which scarlet poppies are interspersed. This section is technically called the pridella. Being small and exactly square in shape it can be easily hinged. See, I pull the cord that controls the locking-catch--thus--and this small window swings open. "Tuesday the twenty-first of June was a warm day, and the pridella in each of the large windows was in use. Now the available aperture is about twenty inches by ten, the glass revolving on central pivots. A boy, or a very small man, might possibly squeeze through, but the bottom ledge of the window being some five feet above the terrace level he would have to use a ladder or a pair of steps in order to reach it. Now, as it chanced, that portion of the lawn lying adjacent to the library terrace was in process of being mowed that morning. I saw the men at work, two of the farm negroes. Assuredly they would have noticed any attempt to scale the windows." "They themselves are quite above suspicion, I suppose." "Unquestionably. They are elderly men who have been employed at the 'Hundred' all their lives, and who bear excellent characters. Zack is the local colored Baptist preacher, and Zeb is an assistant field overseer. Impossible to suspect either, let alone both." "Wouldn't they knock off for dinner at noon? Go to their cabins, I mean."