He handed me back my probe, pocketed his lamp, released the catch of the lock on the door, and turned away along the dark, musty-smelling hall.

“Do you happen to know the name of Johann Schnitzler?” he asked.

I replied that I had no recollection of ever having heard the name before.

“Neither have I,” said he; “but I think we may form a pretty shrewd guess as to his avocation. As you saw, the words ‘Herrn Dr. ’ were printed on the envelope, leaving the rest of the address to be written by hand. The plain inference is that he is a person who habitually addresses letters to medical men, and as the style of the envelope and the lettering⁠—which is printed, not embossed⁠—is commercial, we may assume that he is engaged in some sort of trade. Now, what is a likely trade?”