“Well, that gives us a limit.  Our lady enters this room, and what does she do?  She goes over to the writing-table.  What for?  Not for anything in the drawers.  If there had been anything worth her taking, it would surely have been locked up.  No, it was for something in that wooden bureau.  Halloa! what is that scratch upon the face of it?  Just hold a match, Watson.  Why did you not tell me of this, Hopkins?”

The mark which he was examining began upon the brass-work on the right-hand side of the keyhole, and extended for about four inches, where it had scratched the varnish from the surface.

“I noticed it, Mr. Holmes, but you’ll always find scratches round a keyhole.”

“This is recent, quite recent.  See how the brass shines where it is cut.  An old scratch would be the same colour as the surface.  Look at it through my lens.  There’s the varnish, too, like earth on each side of a furrow.  Is Mrs. Marker there?”

A sad-faced, elderly woman came into the room.

“Did you dust this bureau yesterday morning?”

“Yes, sir.”

“Did you notice this scratch?”

“No, sir, I did not.”

“I am sure you did not, for a duster would have swept away these shreds of varnish.  Who has the key of this bureau?”

“The Professor keeps it on his watch-chain.”

“Is it a simple key?”

“No, sir, it is a Chubb’s key.”

“Very good.  Mrs. Marker, you can go.  Now we are making a little progress.  Our lady enters the room, advances to the bureau, and either opens it or tries to do so.  While she is thus engaged, young Willoughby Smith enters the room.  In her hurry to withdraw the key, she makes this scratch upon the door.  He seizes her, and she, snatching up the nearest object, which happens to be this knife, strikes at him in order to make him let go his hold.  The blow is a fatal one.  He falls and she escapes, either with or without the object for which she has come.  Is Susan, the maid, there?  Could anyone have got away through that door after the time that you heard the cry, Susan?”

“No sir, it is impossible.  Before I got down the stair, I’d have seen anyone in the passage.  Besides, the door never opened, or I would have heard it.”

“That settles this exit.  Then no doubt the lady went out the way she came.  I understand that this other passage leads only to the professor’s room.  There is no exit that way?”

“No, sir.”

“We shall go down it and make the acquaintance of the professor.  Halloa, Hopkins! this is very important, very important indeed.  The professor’s corridor is also lined with cocoanut matting.”

“Well, sir, what of that?”

“Don’t you see any bearing upon the case?  Well, well.  I don’t insist upon it.  No doubt I am wrong.  And yet it seems to me to be suggestive.  Come with me and introduce me.”

We passed down the passage, which was of the same length as that which led to the garden.  At the end was a short flight of steps ending in a door.  Our guide knocked, and then ushered us into the professor’s bedroom.