“All knowledge comes useful to the detective,” remarked Holmes. “Even the trivial fact that in the year 1865 a picture by Greuze entitled ‘La Jeune Fille à l’Agneau’ fetched one million two hundred thousand francs—more than forty thousand pounds—at the Portalis sale may start a train of reflection in your mind.”

It was clear that it did. The inspector looked honestly interested.

“I may remind you,” Holmes continued, “that the professor’s salary can be ascertained in several trustworthy books of reference. It is seven hundred a year.”

“Then how could he buy—”

“Quite so! How could he?”

“Ay, that’s remarkable,” said the inspector thoughtfully. “Talk away, Mr. Holmes. I’m just loving it. It’s fine!”

Holmes smiled. He was always warmed by genuine admiration—the characteristic of the real artist. “What about Birlstone?” he asked.

“We’ve time yet,” said the inspector, glancing at his watch. “I’ve a cab at the door, and it won’t take us twenty minutes to Victoria. But about this picture: I thought you told me once, Mr. Holmes, that you had never met Professor Moriarty.”

“No, I never have.”

“Then how do you know about his rooms?”

“Ah, that’s another matter. I have been three times in his rooms, twice waiting for him under different pretexts and leaving before he came. Once—well, I can hardly tell about the once to an official detective. It was on the last occasion that I took the liberty of running over his papers—with the most unexpected results.”

“You found something compromising?”

“Absolutely nothing. That was what amazed me. However, you have now seen the point of the picture. It shows him to be a very wealthy man. How did he acquire wealth? He is unmarried. His younger brother is a station master in the west of England. His chair is worth seven hundred a year. And he owns a Greuze.”

“Well?”

“Surely the inference is plain.”

“You mean that he has a great income and that he must earn it in an illegal fashion?”

“Exactly. Of course I have other reasons for thinking so—dozens of exiguous threads which lead vaguely up towards the centre of the web where the poisonous, motionless creature is lurking. I only mention the Greuze because it brings the matter within the range of your own observation.”

“Well, Mr. Holmes, I admit that what you say is interesting: it’s more than interesting—it’s just wonderful. But let us have it a little clearer if you can. Is it forgery, coining, burglary—where does the money come from?”

“Have you ever read of Jonathan Wild?”