See that?"
He held out the little piece of mineral I had already seen in his hand
in the alchemist's laboratory.
"That is a piece of willemite. It has the property of glowing or
fluorescing under a certain kind of rays which are themselves
invisible to the human eye. Prescott, your story of the transmutation
of elements is very clever, but not more clever than your real story.
Let us piece it together. I had already heard from Dr. Burnham how Mr.
Haswell was induced by his desire for gain to visit you and how you
had most mysteriously predicted his blindness. Now, there is no such
thing as telepathy, at least in this case. How then was I to explain
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.
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