The constable returned, bearing under his arm a little black leather envelope, and, depositing it on the desk of the Assistant Commissioner, withdrew.
T. B. opened the envelope and removed three neat packages tied with red tape. He unfastened one of these and laid three cards before him. They were three photographic enlargements of a finger print. It did not need the eye of an expert to see they were of the same finger, though it was obvious that they had been made under different circumstances.
T. B. compared them with a smaller photograph he had taken from his pocket. Yes, there was no doubt about it. The four pictures, secured by a delicate process from the almost invisible print on the latest letter of the blackmailer, proved beyond any doubt the identity of Lady Dex's correspondent.
He rang the bell again and the constable appeared in the doorway.
"Is Mr. Ela in his office?"
"Yes, sir. He's been taking information about that Dock case."
"Dock case? Oh yes, I remember; two men were caught rifling the Customs store; they shot a dock constable and got away."
"They both got away, sir," said the man, "but one was shot by the constable's mate; they found his blood on the pavement outside where their motor-car was waiting."
T. B. nodded.