He produced from his pocket a little electric lamp fitted with a bull’s-eye, and, pressing the button, threw a beam of light in through the grille.  The letter was lying on the bottom of the box face upwards, so that the address could easily be read.

“Herrn Dr. H. Weiss,” Thorndyke read aloud.  “German stamp, postmark apparently Darmstadt.  You notice that the ‘Herrn Dr.’ is printed and the rest written.  What do you make of that?”

“I don’t quite know.  Do you think he is really a medical man?”

“Perhaps we had better finish our investigation, in case we are disturbed, and discuss the bearings of the facts afterwards.  The name of the sender may be on the flap of the envelope.  If it is not, I shall pick the lock and take out the letter.  Have you got a probe about you?”

“Yes; by force of habit I am still carrying my pocket case.”

I took the little case from my pocket and extracting from it a jointed probe of thickish silver wire, screwed the two halves together and handed the completed instrument to Thorndyke; who passed the slender rod through the grille and adroitly turned the letter over.

“Ha!” he exclaimed with deep satisfaction, as the light fell on the reverse of the envelope, “we are saved from the necessity of theft—­or rather, unauthorized borrowing—­’Johann Schnitzler, Darmstadt.’  That is all that we actually want.  The German police can do the rest if necessary.”

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?”

“He might be an instrument maker or a drug manufacturer; more probably the latter, as there is an extensive drug and chemical industry in Germany, and as Mr. Weiss seemed to have more use for drugs than instruments.”

“Yes, I think you are right; but we will look him up when we get home.  And now we had better take a glance at the bedroom; that is, if you can remember which room it was.”

“It was on the first floor,” said I, “and the door by which I entered was just at the head of the stairs.”

We ascended the two flights, and, as we reached the landing, I halted.

“This was the door,” I said, and was about to turn the handle when Thorndyke caught me by the arm.