"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."
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 glass.
"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.