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
demonstrated by investigators that blonds are more affected by them
than are brunettes.

"You have, as a part of your machine, a peculiarly shaped quartz
mercury vapor lamp, and the mercury vapor lamp of a design such as
that I saw has been invented for the especial purpose of producing
ultra-violet rays in large quantity. There are also in your machine
induction coils for the purpose of making an impressive noise, and
a small electric furnace to heat the salted gold. I don't know what
other ingenious fakes you have added. The visible bluish light from
the tube is designed, I suppose, to hoodwink the credulous, but the
dangerous thing about it is the invisible ray that accompanies that
light. Mr. Haswell sat under those invisible rays, Prescott, never
knowing how deadly they might be to him, an old man.

"You knew that they would not take effect for hours, and hence you
ventured the prediction that he would be stricken at about midnight.
Even if it was partial or temporary, still you would be safe in your
prophecy. You succeeded better than you hoped in that part of your
scheme. You had already prepared the way by means of a letter sent to
Mr. Haswell through Dr. Burnham. But Mr. Haswell's credulity and fear
worked the wrong way. Instead of appealing to you he hated you. In
his predicament he thought only of his banished daughter and turned
instinctively to her for help. That made necessary a quick change of