Inspector Dunbar stood in the little dispensary tapping his teeth with the end of a fountain-pen.  
"The last time he visited you, doctor—the time when he gave you the envelope—did the cabman wait here in the waiting-room?"
 
"He did—yes. He came after my ordinary consulting hours and I was at supper, I remember, as I am compelled to dine early."
 
"He would be in here alone?"
 
"Yes. No one else was in the room."
 
"Would he have had time to find the box, cut out the piece of cardboard from the lid, put it in the envelope and seal it?"
 
"Ample time. But what could be his object? And why mark the envelope 30?"
 
"It was in your consulting-room that he asked you to take charge of the envelope?"
 
"Yes."
 
"Might I take a peep at the consulting room?"
 
"Certainly, Inspector."
 
From the waiting-room they went up a short flight of stairs into the small apartment in which Stuart saw his patients. Dunbar looked slowly about him, standing in the middle of the room, then crossed and stared out of the window into the narrow lane below.
 
"Where were you when he gave you the envelope?" he snapped suddenly.
 
"At the table," replied Stuart with surprise.
 
"Was the table-lamp alight?"
 
"Yes. I always light it when seeing patients."
 
"Did you take the letter into the study to seal it in the other envelope?"
 
"I did, and he came along and witnessed me do it."
 
"Ah," said Dunbar, and scribbled busily in his note-book. "We are badly tied at Scotland Yard, doctor, and this case looks like being another for which somebody else will reap the credit. I am going to make a request that will surprise you."
 
He tore a leaf out of the book and folded it carefully.
 
"I am going to ask you to seal up something and lock it away! But I don't think you'll be troubled by cowled burglars or beautiful women because of it. On this piece of paper I have written—a"—he ticked off the points on his fingers: "what I believe to be the name of the man who cut out the cardboard and sealed it in an envelope; b: the name of the cabman; and, c: the name of the man who rang me up here last night and gave me information which had only just reached the Commissioner. I'll ask you to lock it away until it's wanted, doctor."
 
"Certainly, if you wish it," replied Stuart. "Come into the study and you shall see me do as you direct. I may add that the object to be served is not apparent to me."
 
Entering the study, he took an envelope, enclosed the piece of paper, sealed the lapel and locked the envelope in the same drawer of the bureau which once had contained that marked 30.
 
"Mlle. Dorian has a duplicate key to this drawer." he said. "Are you prepared to take the chance?"............