“This is where young McFarlane must have come out to get his hat after the crime was done,” said he. “Now look at this.” With dramatic suddenness he struck a match, and by its light exposed a stain of blood upon the whitewashed wall. As he held the match nearer, I saw that it was more than a stain. It was the well-marked print of a thumb.

“Look at that with your magnifying glass, Mr. Holmes.”

“Yes, I am doing so.”

“You are aware that no two thumb-marks are alike?”

“I have heard something of the kind.”

“Well, then, will you please compare that print with this wax impression of young McFarlane’s right thumb, taken by my orders this morning?”

As he held the waxen print close to the blood-stain, it did not take a magnifying glass to see that the two were undoubtedly from the same thumb. It was evident to me that our unfortunate client was lost.

“That is final,” said Lestrade.

“Yes, that is final,” I involuntarily echoed.

“It is final,” said Holmes.

Something in his tone caught my ear, and I turned to look at him. An extraordinary change had come over his face. It was writhing with inward merriment. His two eyes were shining like stars. It seemed to me that he was making desperate efforts to restrain a convulsive attack of laughter.

“Dear me! Dear me!” he said at last. “Well, now, who would have thought it? And how deceptive appearances may be, to be sure! Such a nice young man to look at! It is a lesson to us not to trust our own judgment, is it not, Lestrade?”

“Yes, some of us are a little too much inclined to be cock-sure, Mr. Holmes,” said Lestrade. The man’s insolence was maddening, but we could not resent it.

“What a providential thing that this young man should press his right thumb against the wall in taking his hat from the peg! Such a very natural action, too, if you come to think if it.” Holmes was outwardly calm, but his whole body gave a wriggle of suppressed excitement as he spoke.

“By the way, Lestrade, who made this remarkable discovery?”

“It was the housekeeper, Mrs. Lexington, who drew the night constable’s attention to it.”

“Where was the night constable?”

“He remained on guard in the bedroom where the crime was committed, so as to see that nothing was touched.”

“But why didn’t the police see this mark yesterday?”

“Well, we had no particular reason to make a careful examination of the hall. Besides, it’s not in a very prominent place, as you see.”

“No, no-of course not. I suppose there is no doubt that the mark was there yesterday?”

Lestrade looked at Holmes as if he thought he was going out of his mind. I confess that I was myself surprised both at his hilarious manner and at his rather wild observation.

“I don’t know whether you think that McFarlane came out of jail in the dead of the night in order to strengthen the evidence against himself,” said Lestrade. “I leave it to any expert in the world whether that is not the mark of his thumb.”

“It is unquestionably the mark of his thumb.”

“There, that’s enough,” said Lestrade. “I am a practical man, Mr. Holmes, and when I have got my evidence I come to my conclusions. If you have anything to say, you will find me writing my report in the sitting-room.”

Holmes had recovered his equanimity, though I still seemed to detect gleams of amusement in his expression.

“Dear me, this is a very sad development, Watson, is it not?” said he. “And yet there are singular points about it which hold out some hopes for our client.”

“I am delighted to hear it,” said I, heartily. “I was afraid it was all up with him.”