“But you were insistent that I rub in the—­”

“To force them to wash their hands after touching the towel, Walter.”

“Oh!” I felt rather chagrined.  “Wouldn’t some pigment, some color, have served the purpose better?”

“No, because anyone would have understood that and would have taken the proper measures to remove all traces.  But the itching salve served two purposes.  It was misleading, because obviously a trap upon reflection, and so it would distract attention from the impregnated fibers, my real scheme.  Then it was the best device of all I could think of, for it set up a local irritation of the sort most calculated to make a person clean his finger nails.  The average man and woman is not very neat, Walter.  I was not sure but a scientific prodding was necessary to transfer my evidence to some object I could borrow and examine under a microscope.”

Meanwhile Kennedy’s long fingers were busy at the preliminary operations in his tests.  He turned away and I asked no more questions, not wishing to delay him.

I noticed that first he examined the blood samples under the microscope.  Afterward he employed a spectroscope.  But none of the operations took any great amount of time, since he seemed to anticipate his results.

Mackay burst in upon us, very elated, and produced a handkerchief with a bit of blood upon it.

“I scratched her deliberately with the sharp point of my ring,” he chuckled.  “I found her in the restaurant and the seat beside her was empty.  I—­I talked about everything under the sun and I guess she thinks I’m a clumsy boob!  Anyhow she cried out when I did it, and got red in the face for a moment; but she suspects nothing.”

Kennedy cut the spot from the handkerchief, put it in an envelope, and turned back to his table.  I drew Mackay into the corner.

As the minutes sped by and Craig worked in absorbed concentration, Mackay grew more and more impatient to get back to the studio.

“Did you find anything?” repeated Mackay, for the tenth time.

With a gesture of annoyance, Kennedy reached out for the nail files.

“This is a grave matter,” he frowned.  “I must check it up—­and double check it—­then I’m going back to the studio to triple check it.  Let me see what the nail files reveal.  It will be a bare ten minutes more.”

Insisting that we remain back in the corner, he spread out the four nail files and the open blades of the three pocket knives, setting each upon the envelope which identified it.

The next quarter of an hour seemed interminable.  Finally Kennedy started replacing the files and the pocket knives in their envelopes, his face still wearing the inscrutable frown.  Next he packed the blood samples and other evidence in the traveling bag once more.

Mackay was bursting with impatience, but Craig still refused to betray his suspicions.

“I must get back there—­quick,” he hastened.  “I want everybody in the projection room.  In court, a jury might not grasp the infallibility of the methods I’ve used.  There would be a great deal of medical and expert testimony required—­and you know, Mackay, what that means.”