The idea tickled Gregson so much that he laughed until he choked.

“And how did you get your clue?”

“Ah, I’ll tell you all about it. Of course, Doctor Watson, this is strictly between ourselves. The first difficulty which we had to contend with was the finding of this American’s antecedents. Some people would have waited until their advertisements were answered, or until parties came forward and volunteered information. That is not Tobias Gregson’s way of going to work. You remember the hat beside the dead man?”

“Yes,” said Holmes; “by John Underwood and Sons, 129, Camberwell Road.”

Gregson looked quite crestfallen.

“I had no idea that you noticed that,” he said. “Have you been there?”

“No.”