The light exposes a stain of blood upon the whitewashed wall.It is 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?”