it? What could cause such a catastrophe naturally? Why, only those
rays invisible to the human eye, but which make this piece of
willemite glow--the ultra-violet rays."

Kennedy was speaking rapidly and was careful not to pause long enough
to give Prescott an opportunity to interrupt him.

"These ultra-violet rays," he continued, "are always present in an
electric arc light though not to a great degree unless the carbons
have metal cores. They extend for two octaves above the violet of the
spectrum and are too short to affect the eye as light, although they
affect photographic plates. They are the friend of man when he uses
them in moderation as Finsen did in the famous blue light treatment.
But they tolerate no familiarity. To let them--particularly the
shorter of the rays--enter the eye is to invite trouble. There is no
warning sense of discomfort, but from six to eighteen hours after
exposure to them the victim experiences violent pains in the eyes and
headache. Sight may be seriously impaired, and it may take years to
recover. Often prolonged exposure results in blindness, though a
moderate exposure acts like a tonic. The rays may be compared in this
double effect to drugs, such as strychnine. Too much of them may be
destructive even to life itself."

Prescott had now paused and was regarding Kennedy contemptuously.
Kennedy paid no attention, but continued: "Perhaps these mysterious
rays may shed some light on our minds, however. Now, for one thing,
ultra-violet light passes readily through quartz, but is cut off by
ordinary glass, especially if it is coated with chromium. Old Mr.
Haswell did not wear glasses. Therefore he was subject to the
rays--the more so as he is a blond, and I think it has been