“He remained on guard in the bedroom where the crime was committed, so as to see that nothing was touched.”

“But why didn’t the police see this mark yesterday?”

“Well, we had no particular reason to make a careful examination of the hall. Besides, it’s not in a very prominent place, as you see.”

“No, no⁠—of course not. I suppose there is no doubt that the mark was there yesterday?”

Lestrade looked at Holmes as if he thought he was going out of his mind. I confess that I was myself surprised both at his hilarious manner and at his rather wild observation.

“I don’t know whether you think that McFarlane came out of jail in the dead of the night in order to strengthen the evidence against himself,” said Lestrade. “I leave it to any expert in the world whether that is not the mark of his thumb.”

“It is unquestionably the mark of his thumb.”