Who had done this thing? Who had sought, with such patient cunning, to upset those evidential principles by which blind Justice gropes her hesitating way to Truth? In concocting his masterpiece of malignant ingenuity the murderer had worked alone. His only accomplice--apart from the after-hand of Fate--was a piece of automatic mechanism which had done his bidding secretly, and would never have betrayed him. It was this ability to work alone, scheming and brooding in solitary concentration until the whole of the horrible conception had been perfected in every degree, which stamped the designer as a ferocious criminal of unusual mould, remorseless as a tiger, with a neurasthenic mind swayed by the unbridled savagery of natural impulse. As Colwyn meditated over the murder, his original impression of the guests assembled in the dining-room downstairs in a premeditated scene set for its production came back to him with renewed force. The murderer had taken his part in that scene as one of the unconscious audience, dining and taking his share in the conversation, while his secret consciousness was strained to an intense anticipation of the false signal from his mechanical accomplice upstairs. Colwyn could picture him joining in the mockery of meaningless phrases with dry lips, his ears listening for every sound, his eyes covertly watching the crawling hands of the clock. Then, when the crack had pealed forth, he had been able to exchange suspense for action, and rush upstairs with the others, confident in the feeling that, let suspicion point where it would, it could not fall on him. But the murderer had not foreseen the scream which preceded the shot. How had he comported himself under the shock of that cry, which was outside the region of his calculations? He had not time to reflect upon its origin, to investigate its source. He had to steel his nerves to face it because he dared not do otherwise. But its sudden effect on the nerve centres of his brain, previously strained almost to the breaking point, must have brought him to the verge of a subsequent collapse. Colwyn believed he saw the end in sight. The presumptions, the facts, and the motive all pointed to one figure as the murderer of Violet Heredith. She had been killed from the dual motive of punishment in her own case and vengeance on a greater offender than herself. The alibi had been devised to ensure a tremendous revenge on the man by bringing him to the gallows as her supposed murderer. That part of the plan had gone astray, so the murderer, in the fanatical resolve of his latent fixed idea, had recourse to a further expedient as daring and original as the scheme which failed. The second instrument had been the means of his own undoing.