Lockwood rang off, and I repeated what he had told me, as Kennedy continued to adjust the apparatus.

“Say,” I exclaimed, as I finished.  “That was a harry’s of a commission you gave Norton just now, watching the de Moches.  Why, they’d eat him alive if they got a chance, and I don’t know that all’s like a Sunday school on his part.  Lockwood doesn’t seem to think so.”

Kennedy smiled quietly.  “That was why I asked him to do it,” he returned.  “I thought that he wouldn’t let much escape him.  They all seem so down on him, he’ll have to watch out.  It will keep him busy, too, and that means a chance for us to work.”

He had finished setting up the machine, and now went over to another drawer, from which he took the envelope of stubs which we had taken down at Whitney’s office first.  Then from the pocket of his street coat he drew both the second envelope of ashes and stubs, the whole cigarette from Lockwood’s case, and the stubs which both of us had saved from the cigarettes that had once belonged to Mendoza.

Carefully he separated and labelled them all, so that there would be no chance for them to get mixed up.  Then he picked up one of the stubs and lighted it.  The smoke curled up in wreaths between a powerful light and the peculiar instrument, while Craig peered through a lens, manipulating the thing with exhaustless patience and skill.  I watched him curiously, but said nothing, for he was studying something carefully, and I did not want to interrupt his train of thought.

Finally he beckoned me over.  “Can you make anything out of that?” he asked.

I looked through the eye-piece, also.  On a sort of fine grating all I could see was a number of strange lines.

“If you want an opinion from me,” I said, with a laugh, “you’ll have to tell me first what I am looking at.”

“That,” he explained, as I continued to gaze, “is one of the latest forms of the spectroscope, known as the interferometer, with delicately ruled gratings in which power to resolve the straight, close lines in the spectrum is carried to the limit of possibility.  A small watch is delicate.  But it bears no comparison to the delicacy of these defraction spectroscopes.

“Every substance, you know, is, when radiating light, characterized by what at first appears to be almost haphazard sets of spectral bands without relation to one another.  But they are related by mathematical laws, and the apparent haphazard character is only the result of our lack of knowledge of how to interpret the results.”

He resumed his place at the eye-piece to check over his results.

“Walter,” he said finally, looking up at me with a twinkle in his eye, “I wish that you’d go out and find me a cat.”

“A cat?” I repeated.

“Yes, a cat—­felis domesticus, if it sounds better that way—­a plain, ordinary cat.”

I jammed on my hat and, late as it was, sallied forth on this apparently ridiculous mission.