"Precisely. It had been replaced in a new position, an entirely unknown one. As it happened--pure chance, you understand--the ray of sunshine that fell upon your face at noon that day had passed through a bullseye of common purple glass, and therefore it was harmless. But the Terror was in the room; somewhere it was lying in wait, ready to strike. Do you recall how I kept close to the wall, so as to avoid getting in the path of the direct sunlight? You understand now that I realized the danger, and took the obvious precaution. But John Thaneford was unaware that any change had been made in the position of the death-dealing lens. And so he walked straight into the line of destructive force; and the Sigma ray, being no respecter of persons, proceeded to strike him down." "I wonder how much he really knew about the whole affair?" queried Betty. "You remember that Eunice expressly acquitted John Thaneford of any actual part in my father's death." "But he certainly must have been cognizant of the nature of the trap," answered Warriner. "He was the observer at the time of Mr. Graeme's death, the elder Thaneford being physically unable to take his accustomed post on Sugar Loaf. Again, his putting Hugh, bound and helpless, into the fatal chair is unanswerable evidence that he did possess a guilty knowledge of his father's secret. It makes no moral difference that he had no hand in inventing or setting up the instrument of vengeance. He knew of its existence undoubtedly, and hoped to profit by it. That's enough." "Have you any theory about the Sigma ray itself?" I asked. "Or rather its effect upon the physical organism?" "Do you happen to recall the medical testimony given at the coroner's inquest by Doctor Williams of John Hopkins? Well, he testified, in brief, that the autopsy had revealed a most peculiar lesion of the brain; in unprofessional language, the injury might be characterized as a case of greatly intensified sunstroke." "Yes, I do remember." "Now there are unexplained anomalies about even ordinary sunstroke," continued Warriner. "Just what are the conditions under which exposures to the rays of the sun may be dangerous? "In the first place, we may affirm confidently that the peril is not dependent upon the amount of humidity that may be present in the atmosphere. Down in New Orleans, where the air is full of moisture and the thermometer stands high in the scale for weeks at a time, sunstroke is virtually unknown; men and beasts seem equally immune. But let a ten-day heat wave submerge New York City and the emergency hospitals will be full up, while the horses will be wearing plaited straw-bonnets as a protection against the deadly sun.