"Doctor Nash had employed me to travel with Paul and keep him under observation, and it was thought wiser for Zybinn not to communicate directly with me," Roberts turned to Miriam. "A glass of water, please." Riley got it for him, before Miriam could move, from the pitcher placed for Mrs. Nash's use on the bedstand. Roberts looked over at Betty, a malignant grin distorting his face. "Zybinn used you as a cat's paw," he said. "Through you he gained an intimate knowledge of Paul's habits, his mode of life, and, using his remarkable powers of deduction, twice located the hiding place of the diamond--in each instance too late, for Paul's capricious habits, his secretiveness, yes," with grudging admiration, "his cleverness balked us. And so did you," wheeling on Mrs. Nash with a suddenness which made her jump. "I tried to secure the thirteenth letter on Tuesday night, but Martha detected me, and last night you pulled off my disguise." "Why did you risk discovery?" asked Mrs. Nash. "Why not have telegraphed to Zybinn for the message on his last letter?" "I telephoned from Washington on Tuesday and was told he had died from apoplexy on Monday afternoon--his third stroke," added Roberts. "That message on his letter to Paul was Zybinn's last word to me. He thought I was still here at Abbott's Lodge." "Just a moment," broke in Trenholm. "Why did Zybinn use the words 'thirteenth letter' to designate the initial 'M' on the back of the watch?" "Because in devising our code we failed to make provision for indicating an initial, expecting never to use one." Roberts chafed one cold hand over the other. "Had I decoded Zybinn's last message, I'd have gotten his meaning, however, for that little sketch is the only painting by Paul's mother on the premises and always cherished by her son. He invariably spoke of the sketch as 'The Suicide's Grave.'" "I told Zybinn that," admitted Betty. "Great heavens! how I played into his hands--" "Just so!" agreed Roberts with sneering emphasis. He straightened up, swayed slightly and recovered his balance with an effort. "Come," addressing Trenholm, "I can stand no more." The constable was by his side and Riley at his heels instantly. "We'll take him to Upper Marlboro, sir," the former stated, and at a nod from Trenholm, Roberts, with eyes averted from his former friends, left the room, the black shroudlike cloth still thrown about his shoulders--typical in its vague outlines of the shadowed and complex nature of the man. Mrs. Nash's overcharged feelings found relief in tears. "There," she exclaimed, as her distracted husband held a glass of water and Miriam the smelling salts. "I'll be myself in a minute. Betty, come and tell me why you remained here, instead of returning to Washington with your uncle, and why you lied about your visit to Paul." Betty cleared her throat. "You were partly responsible--" "I?" her aunt regarded her in astonishment.