Dillon, I found, was overjoyed when I reported to him the capture of at least one man whom we had failed to get the night before.

"Things seem to be clearing up, after all," he remarked. "Tell Garrick I shall hold open to-night for him. Meanwhile, good luck, and let me know the moment you get any word about the Chief. He must have been in.

that first cab, all right."

As I left Dillon's office, I ran into Herman in the hall, coming in. I bowed to him and he nodded surlily. Evidently, I thought, he had heard of the result of our activities. I did not ask him what progress he had made in the case, for I had had experience with professional jealousy before, and thought that the less said on the subject the better.

Recalling what Garrick had said, I curbed my impatience as best I could, in order to give him ample time to complete the work that he had to do. It was not until the middle of the afternoon that I rejoined him in his office.

I found him at work at a table, still, with a microscope and an arrangement which I recognised as the apparatus for making microphotographs. Several cartridges, carefully labelled, were lying before him, as well as the peculiar pistol we had found when we had captured Forbes in the little room. There were also the guns we had captured in the garage and one found in the cab which we had chased and wrecked.

On the end of the table was a large number of photographs of a most peculiar nature. I picked up one. It looked like an enlarged photograph of an orange, or like some of the pictures which the astronomers make of the nearer planets.

"What are these?" I asked curiously, as he leaned back from his work, with a smile of quiet satisfaction.

"That is a collection of microphotographs which I have gathered," he answered, adding, "as well as some that I have just made. I hope to use them in a little stereopticon entertainment I am arranging to-night for those who have been interested in the case."

Garrick smiled. "Have you ever heard?" he asked, "that the rounded end of the firing pin of every rifle when it is examined under a microscope bears certain irregularities of marking different from those of every other firing pin and that the primer of every sh.e.l.l fired in a rifle is impressed with the particular markings of that firing pin?"

I had not, but Garrick went on, "I know that it is true. Such markings are distinctive for each rifle and can be made by no other. I have taken rifles bearing numbers preceding and following that of a particular one, as well as a large number of other firing pins. I have tried the rifles and the firing pins, one by one, and after I made microphotographs of the firing pins with special reference to the rounded ends and also photographs of the corresponding rounded depressions in the primers fired by them, it was forced upon me that cartridges fired by each individual firing pin could be positively identified."

I had been studying the photographs. It was a new idea, and it appealed to me strongly. "How about revolvers?" I asked quickly.

"Well, Dr. Balthazard, the French criminologist, has made experiments on the identification of revolver bullets and has a system that might be compared to that of Bertillon for identifying human beings. He has showed by greatly enlarged photographs that every gun barrel leaves marks on a bullet and that the marks are always the same for the same barrel but never identical for two different barrels. He has shown that the hammer of a revolver, say a centre fire, strikes the cartridge at a point which is never the exact centre of the cartridge, but is always the same for the same weapon. He has made negatives of bullets nearly a foot wide. Every detail appears very distinctly and it can be decided with absolute certainty whether a certain bullet or cartridge was fired by a certain revolver."

He had picked up one of the microphotographs and was looking at it attentively through a small gla.s.s.

"You will see," he explained, "on the edge of this photograph a rough sketch calling attention to a mark like an L which is the chief characteristic of this hammer, although there are other detailed markings which show well under the microscope but not in a photograph.

You will note that the marks on a hammer are reversed on the primer in the same way that a metal type and the character printed by it are reversed as regards one another. Moreover, depressions on the end of a hammer become raised on the primer and raised markings on the hammer become depressions on the primer.

"Now, here is another. You can see that it is radically different from the first, which was from the cartridge used in killing poor Rena Taylor. This second one is from that gun which I found on the tenement roof this morning. It lacks the L mark as well as the concentric circles. Here is another. Its chief characteristics are a series of pits and elevations which, examined under the microscope and measured, will be found to afford a set of characters utterly different from those of any other hammer.

"In short," he concluded with an air of triumph, "the ends of firing pins are turned and finished in a lathe by the use of tools designed for that purpose. The metal tears and works unevenly so that microscopical examination shows many pits, lines, circles, and irregularities. The laws of chance are as much against two of these firing pins or hammers having the same appearance under the microscope as they are against the thumb prints of two human subjects being identical."

I picked up the curious little arrangement which we had found in the drawer in Forbes' room and examined it closely.

"I have been practicing with that pistol, if you may call it that," he remarked, "on cartridges of my own and examining the marks made by the peculiar hammer. I have studied marks of the gun which we found on the roof. I have compared them with the marks on cartridges which we have picked up at the finding of Rena Taylor's body, at the garage that night of the stupefying bullet, with bullets such as were aimed at Warrington, with others, both cartridges and bullets, at various times, and the conclusion is unescapable."

Who, I asked myself, was the scientific gunman? I knew it was useless to try to hurry Garrick. First, by a sort of intuition he had picked him out, then by the evidence of hammer and bullet he had made it practically certain. But I knew that to his scientific mind nothing but absolute certainty would suffice.

While I was waiting for him to proceed, he had already begun to work on some apparatus behind a screen at the end of his office. Close to the wall at the left was a stereopticon which, as nearly as I could make out, shot a beam of light through a tube to a galvanometer about three feet distant. In front of this beam whirled a five-spindled wheel governed by a chronometer which was so accurate, he said, that it erred only a second a day.

Between the poles of the galvanometer was stretched a slender thread of fused quartz plated with silver. It was the finest thread I could imagine, only a thousandth of a millimeter in diameter, far too tenuous to be seen. Three feet further away was a camera with a moving plate holder which carried a sensitized photographic plate. Its movement was regulated by a big fly-wheel at the extreme right.

"You see," remarked Garrick, now engrossed on the apparatus and forgetting the hammer evidence for the time, "the beam of light focussed on that fine thread in the galvanometer pa.s.ses to this photographic plate. It is intercepted by the five spindles of the wheel, which turns once a second, thus marking the picture off in exact fifths of a second. The vibrations of the thread are enormously magnified on the plate by a lens and produce a series of wavy or zigzag lines. I have shielded the sensitized plate by a wooden hood which permits no light to strike it except the slender ray that is doing the work. The plate moves across the field slowly, its speed regulated by the fly-wheel. Don't you think it is neat and delicate? All these movements are produced by one of the finest little electric motors I ever saw."

I could not get the idea of the revolvers out of my head so quickly. I agreed with him, but all I could find to say was, "Do you think there was more than this one whom they call the Chief engaged in the shootings?"

"I can't say absolutely anything more than I have told you, yet," he answered in a tone that seemed to discourage further questioning along that line.

He continued to work on the delicate apparatus with its thread stretched between the stationary magnets of the galvanometer, a thread so delicate that it might have been spun by a microscopic spider, so light that no scales made by human hands could weigh it, so slender that the mind could hardly grasp it. It was about one-third the diameter of a red corpuscle of blood and its weight had been estimated as about .00685 milligrams, truly a fairy thread. It was finer than the most delicate cobweb and could be seen with the naked eye only when a strong light was thrown on it so as to catch the reflection.

"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 sh.e.l.l 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.

"I have made a collection from time to time," he pursued, "of the various exploded cartridges, the bullets, and the weapons left behind by the perpetrator of the dastardly series of crimes, from the shooting of the stool pigeon of the police, Rena Taylor, and the stealing of Mr.

Warrington's car, down to the peculiar events of last night up in the Ramapos and the running fight through the streets of New York in taxicabs this morning.

"I have studied this evidence with the microscope and the microphotographic apparatus. I have secured excellent microphotographs of the marks made by various weapons on the cartridges and bullets.

Taking those used in the commission of the greater crimes in this series, I find that the marks are the same, apparently, whether the gun shot off a bullet of wax or tallow which became liquid in the body, whether it discharged a stupefying gas, or whether the deadly anaesthetic bullet was fired. I have obtained a gun"--he threw it on the table with a clang--"the marks from the hammer of which correspond with the marks made on all the cartridges I have mentioned. One person owned that gun and used it. That is proved. It remains only to connect that gun positively and definitely, as a last link, with that person."

I noticed with a start that the revolver still had a stout cord tied to it.

As he concluded, Garrick had begun fitting a curious little device to each of our forearms. It looked to me like an electrode consisting of large plates of German silver, covered with felt and saturated with salt solution. From each electrode wires ran across the floor to some hidden apparatus.

"Back of this screen," he went on, indicating it in the corner of the room, "I have placed what is known as the string galvanometer, invented, or, perhaps better, perfected by Dr. Einthoven, of Leyden. It was designed primarily for the study of the beating of the heart in cases of disease, but it also may be used to record and study emotions as well,--love and hate, fear, joy, anger, remorse, all are revealed by this uncanny, cold, ruthlessly scientific instrument.

"The machine is connected by wires to each of you, and will make what are called electrocardiographs, in which every emotion, every sentiment, every pa.s.sion is recorded inevitably, inexorably. For, the electric current that pa.s.ses from each of you to the machine over these wires carrying the record of the secrets of your hearts is one of the feeblest currents known to science. Yet it can be caught and measured.

The dynamo which generates this current is not a huge affair of steel castings and endless windings of copper wire. It is merely the heart of the sitter.

"The heart makes only one three-thousandth of a volt of electricity at each beat. It would take thousands of hearts to light one electric light, hundreds of thousands to run one trolley car. Yet just that slight little current from the heart is enough to sway a gossamer strand of quartz fibre in what I may call my 'heart station' here. This current, as I have told you, pa.s.ses from each of you over a wire and vibrates a fine quartz fibre in unison with it, one of the most delicate bits of mechanism ever made, recording the result on a photographic film by means of a beam of light reflected from a delicate mirror."

We sat spellbound as Garrick unfolded the dreadful, awe-inspiring possibilities of the machine behind the screen. He walked slowly to the back of the room.

"Now, here I have one of the latest of the inventions of the Wizard of West Orange--Edison," he resumed. "It is, as you perhaps have already guessed, the latest product of this genius of sound and sight, the kinetophone, the machine that combines moving pictures with the talking machine."