I was disappointed.  It seemed to me that he had made comparatively little progress so far.

“There’s one thing,” he added.  “Samples of the body fluids of the victim have been sent down by the coroner at Tarrytown and I have analyzed them.  While I haven’t decided what it was that killed Stella Lamar, I am at least convinced that it has something to do with these towel spots.  They are not exactly the same—­in fact, I should say they were complementary, or, perhaps better, antithetical.”

“The mark wasn’t made by the needle which scratched her, then?”

“That’s what I thought at first, that the point used had been wiped off on the towel.  Then I decided that the spots had nothing to do with the case at all.  Now I believe there is some connection, after all.”

“I—­I don’t understand it,” I protested.

“It’s very baffling,” he agreed, absent-mindedly.

“If the towel wasn’t used to clean the fatal needle,” I went on, “then it may have been used before they went out instead of afterward.”

“Exactly.  As a matter of fact, if I had not been so confused yesterday by all the details of the case, by the many people involved, I would have noticed at a glance that the blood spots on the towel could not come from some one using it to wipe the needle.  And any hypothesis that it had been used out in Tarrytown was ridiculous, because Miss Lamar was only scratched faintly and lost no blood.  If I had been a little more clever I might have been altogether too clever.  I might possibly have thrown the towel away, because there certainly was no logical reason for connecting it with the crime.”

“Just when do you suppose Stella was pricked?” I asked.

“That’s a vital consideration.  Just now I do not know the poison and so cannot tell how quickly it acted.”  He began to put aside his various paraphernalia.  “Suppose we go at this thing by a process of deduction rather than from the end of scientific analysis.”  He sat on a corner of the bench.  “What do we find?” he began.

“While I’ve been working here with the test tubes and the microscope I’ve been trying to reconstruct what must have happened, trying to trace out every action of Stella Lamar as nearly as it is possible for us to do so.  I don’t think we need to go back of their arrival at the house, for the present.  They seem to have been there a long while before the taking of the particular scene, since there were twelve other scenes preceding and since it requires time to put up the electric lights and make the connections, as well as to set the cameras, take tests, rearrange the furniture, and all the rest of it.