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“It shows eight teeth, one of them decayed,” he remarked. “By the way, there’s no use watching here any longer. I have some more work to do in the laboratory which will keep me another day. To- morrow night I shall be ready. Andrews, in the mean time I leave the shadowing of Dana to you, and with the help of Jameson I want you to arrange to have all those connected with the case at my laboratory to-morrow night without fail.”

Andrews and I had to do some clever scheming to bring pressure to bear on the various persons interested to insure their attendance, now that Craig was ready to act. Of course there was no difficulty in getting Dana Phelps. Andrews’s shadows reported nothing in his actions of the following day that indicated anything. Mrs. Phelps came down to town by train and Doctor Forden motored in. Andrews even took the precaution to secure Shaughnessy and the trained nurse, Miss Tracy, who had been with Montague Phelps during his illness but had not contributed anything toward untangling the case. Andrews and myself completed the little audience.

We found Kennedy heating a large mass of some composition such as dentists use in taking impressions of the teeth.

“I shall be ready in a moment,” he excused himself, still bending over his Bunsen flame. “By the way, Mr. Phelps, if you will permit me.”

He had detached a wad of the softened material. Phelps, taken by surprise, allowed him to make an impression of his teeth, almost before he realised what Kennedy was doing. The precedent set, so to speak, Kennedy approached Doctor Forden. He demurred, but finally consented. Mrs. Phelps followed, then the nurse, and even Shaughnessy.

With a quick glance at each impression, Kennedy laid them aside to harden.

“I am ready to begin,” he remarked at length, turning to a peculiar looking instrument, something like three telescopes pointing at a centre in which was a series of glass prisms.

“These five senses of ours are pretty dull detectives sometimes,” Kennedy began. “But I find that when we are able to call in outside aid we usually find that there are no more mysteries.”

He placed something in a test-tube in line before one of the barrels of the telescopes, near a brilliant electric light.

“What do you see, Walter?” he asked, indicating an eyepiece.

I looked. “A series of lines,” I replied. “What is it?”

“That,” he explained, “is a spectroscope, and those are the lines of the absorption spectrum. Each of those lines, by its presence, denotes a different substance. Now, on the pavement of the Phelps mausoleum I found, you will recall, some roundish spots. I have made a very diluted solution of them which is placed in this tube.

“The applicability of the spectroscope to the differentiation of various substances is too well known to need explanation. Its value lies in the exact nature of the evidence furnished. Even the very dilute solution which I have been able to make of the material scraped from these spots gives characteristic absorption bands between the D and E lines, as they are called. Their wave- lengths are between 5774 and 5390. It is such a distinct absorption spectrum that it is possible to determine with certainty that the fluid actually contains a certain substance, even though the microscope might fail to give sure proof. Blood– human blood–that was what those stains were.”

He paused. “The spectra of the blood pigments,” he added, “of the extremely minute quantities of blood and the decomposition products of hemoglobin in the blood are here infallibly shown, varying very distinctly with the chemical changes which the pigments may undergo.”

Whose blood was it? I asked myself. Was it of some one who had visited the tomb, who was surprised there or surprised some one else there? I was hardly ready for Kennedy’s quick remark.

“There were two kinds of blood there. One was contained in the spots on the floor all about the mausoleum. There are marks on the arm of Dana Phelps which he probably might say were made by the teeth of my police-dog, Schaef. They are human tooth-marks, however. He was bitten by some one in a struggle. It was his blood on the floor of the mausoleum. Whose were the teeth?”