“All I can say is,” he admitted, “that the bullets which committed this horrible series of crimes have been proven all to be shot from the same gun, presumably, I think I shall show, by the same hand, and that hand is the same that wrote the blackmailing letter.”

“Whose gun was it?” I asked.  “Was there a way to connect it and the bullets and the cartridges with the owner—­four things, all separated—­and then that owner with the curious and tragic succession of events that had marked the case since the theft of Warrington’s car?”

Garrick had apparently completed his present work of adjusting the delicate apparatus.  He was now engaged on another piece which also had a powerful light in it and an attachment which bore a strong resemblance to a horn.

He paused a moment, regarding me quizzically.  “I think you’ll find it sufficiently novel to warrant your coming, Tom,” he added.  “I have already invited Dillon and his man, Herman, over the telephone just before you came in.  McBirney will be there, and Forbes, of course.  He’ll have to come, if I want him.  By the way, I wish you’d get in touch with Warrington and see how he is.  If it is all right, tell him that I’d like to have him escort Miss Winslow and her aunt here, to-night.  Meanwhile I shall find out how our friend the Boss is getting on.  He ought to be here, at any cost, and I’ve put it off until to-night to make sure that he’ll be in fit condition to come.  To-night at nine—­here in this office—­remember,” he concluded gayly.  “In the meantime, not a word to anybody about what you have seen here this afternoon.”

CHAPTER XXV

THE SCIENTIFIC GUNMAN

Our little audience arrived one by one, and, as master of ceremonies, it fell to me to greet them and place them as much at ease as the natural tension of the occasion would permit.  Garrick spoke a word or two to each, but was still busy putting the finishing touches on the preparations for the “entertainment,” as he called it facetiously, which he had arranged.

“Before I put to the test a rather novel combination which I have arranged,” began Garrick, when they had all been seated, “I want to say a few words about some of the discoveries I have already made in this remarkable case.”

He paused a moment to make sure that he had our attention, but it was unnecessary.  We were all hanging eagerly on his words.

“There is, I believe,” he resumed slowly, “no crime that is ever without a clew.  The slightest trace, even a drop of blood no larger than a pin-head, may suffice to convict a murderer.  So may a single hair found on the clothing of a suspect.  In this case,” he added quickly, “it is the impression made by the hammer of a pistol on the shell of a cartridge which leads unescapably to one conclusion.”

The idea was so startling that we followed Garrick’s every word as if weighted with tremendous importance, as indeed it was in the clearing up of this mysterious affair.