The pleasant, open face of our visitor clouded over, shadowed by some grim remembrance. “What I speak of occurred the night before last,” said he. “I was lying awake about two in the morning, when I was aware of a dull muffled sound coming from the passage. I opened my door and peeped out. I should explain that the professor sleeps at the end of the passage⁠—”

“The date being⁠—?” asked Holmes.

Our visitor was clearly annoyed at so irrelevant an interruption.

“I have said, sir, that it was the night before last⁠—that is, September 4th.”

Holmes nodded and smiled.

“Pray continue,” said he.