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 shell 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.”