“Which tong does he belong to?” asked Kennedy, still scrutinising a photograph through his lens.

“Neither,” replied O’Connor.  “With his aid and that of a Judge of one of our courts who knows the Chinaman like a book we have had a conference this afternoon between the two tongs and the truce is restored again for two weeks.”

“Very good,” answered Kennedy, “but it doesn’t catch the murderer of Bertha Curtis and the Jap.  Where is Clendenin, do you suppose?”

“I don’t know, but it at least leaves me free to carry on that case.  What are all these pictures?”

“Well,” began Kennedy, taking his glass from his eye and wiping it carefully, “a Paris crime specialist has formulated a system for identifying revolver bullets which is very like that of Dr. Bertillon for identifying human beings.”

He picked up a handful of the greatly enlarged photographs.  “These are photographs of bullets which he has sent me.  The barrel of every gun leaves marks on the bullet that are always the same for the same barrel but never identical for two different barrels.  In these big negatives every detail appears very distinctly and it can be decided with absolute certainty whether a given bullet was fired from a given revolver.  Now, using this same method, I have made similar greatly enlarged photographs of the two bullets that have figured so far in this case.  The bullet that killed Miss Curtis shows the same marks as that which killed Nichi.”

He picked up another bunch of prints.  “Now,” he continued, “taking up the firing pin of a rifle or the hammer of a revolver, you may not know it but they are different in every case.  Even among the same makes they are different, and can be detected.

“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.