"The cartridge in either a gun or revolver is struck at a point which is never in the exact centre or edge, as the case may be, but is always the same for the same weapon. Now the end of the hammer when examined with the microscope bears certain irregularities of marking different from those of every other gun and the shell fired in it is impressed with the particular markings of that hammer, just as paper is by type. On making microphotographs of firing pins or hammers, with special reference to the rounded ends and also photographs of the corresponding rounded depressions in the primers fired by them it is forced on any one that cartridges fired by each individual rifle or pistol can positively be identified.
"You will see on the edge of the photographs I have made a rough sketch calling attention to the 'L'-shaped mark which is the chief characteristic of this hammer, although there are other detailed markings which show well under the microscope but not well in a photograph. You will notice that the characters on the firing hammer are reversed on the cartridge in the same way that a metal type and the character printed by it are reversed as regards one another. Again, depressions on the end of the hammer become raised characters on the cartridge, and raised characters on the hammer become depressions on the cartridge.
"Look at some of these old photographs and you will see that they differ from this. They lack the 'L' mark. Some have circles, others a very different series of pits and elevations, a set of characters when examined and measured under the microscope utterly
