“There’s the advantage of being a builder,” said Holmes, as we came out.  “He was able to fix up his own little hiding-place without any confederate—­save, of course, that precious housekeeper of his, whom I should lose no time in adding to your bag, Lestrade.”

“I’ll take your advice.  But how did you know of this place, Mr. Holmes?”

“I made up my mind that the fellow was in hiding in the house.  When I paced one corridor and found it six feet shorter than the corresponding one below, it was pretty clear where he was.  I thought he had not the nerve to lie quiet before an alarm of fire.  We could, of course, have gone in and taken him, but it amused me to make him reveal himself.  Besides, I owed you a little mystification, Lestrade, for your chaff in the morning.”

“Well, sir, you certainly got equal with me on that.  But how in the world did you know that he was in the house at all?”

“The thumb-mark, Lestrade.  You said it was final; and so it was, in a very different sense.  I knew it had not been there the day before.  I pay a good deal of attention to matters of detail, as you may have observed, and I had examined the hall, and was sure that the wall was clear.  Therefore, it had been put on during the night.”

“But how?”

“Very simply.  When those packets were sealed up, Jonas Oldacre got McFarlane to secure one of the seals by putting his thumb upon the soft wax.  It would be done so quickly and so naturally, that I daresay the young man himself has no recollection of it.  Very likely it just so happened, and Oldacre had himself no notion of the use he would put it to.  Brooding over the case in that den of his, it suddenly struck him what absolutely damning evidence he could make against McFarlane by using that thumb-mark.  It was the simplest thing in the world for him to take a wax impression from the seal, to moisten it in as much blood as he could get from a pin-prick, and to put the mark upon the wall during the night, either with his own hand or with that of his housekeeper.  If you examine among those documents which he took with him into his retreat, I will lay you a wager that you find the seal with the thumb-mark upon it.”

“Wonderful!” said Lestrade.  “Wonderful!  It’s all as clear as crystal, as you put it.  But what is the object of this deep deception, Mr. Holmes?”

It was amusing to me to see how the detective’s overbearing manner had changed suddenly to that of a child asking questions of its teacher.

“Well, I don’t think that is very hard to explain.  A very deep, malicious, vindictive person is the gentleman who is now waiting us downstairs.  You know that he was once refused by McFarlane’s mother?  You don’t!  I told you that you should go to Blackheath first and Norwood afterwards.  Well, this injury, as he would consider it, has rankled in his wicked, scheming brain, and all his life he has longed for vengeance, but never seen his chance.