In a few short sentences he explained to the inspector the factsabout the letter and the cipher. MacDonald sat with his chin on hishands and his great sandy eyebrows bunched into a yellow tangle.
“I was going down to Birlstone this morning，” said he. “I hadcome to ask you if you cared to come with me—you and yourfriend here. But from what you say we might perhaps be doingbetter work in London.”
“I rather think not，” said Holmes.
“Hang it all， Mr. Holmes！” cried the inspector. “The papers willbe full of the Birlstone mystery in a day or two； but where’s themystery if there is a man in London who prophesied the crimebefore ever it occurred？ We have only to lay our hands on thatman， and the rest will follow.”
“No doubt， Mr. Mac. But how do you propose to lay your handson the so-called Porlock？”
MacDonald turned over the letter which Holmes had handedhim. “Posted in Camberwell—that doesn’t help us much. Name，you say， is assumed. Not much to go on， certainly. Didn’t you saythat you have sent him money？”
“Twice.”
“And how？”
“In notes to Camberwell post office.”
“Did you ever trouble to see who called for them？”
“No.”
The inspector looked surprised and a little shocked. “Why not？”
“Because I always keep faith. I had promised when he firstwrote that I would not try to trace him.”
“You think there is someone behind him？”
“I know there is.”
“This professor that I’ve heard you mention？”
“Exactly！”
Inspector MacDonald smiled， and his eyelid quivered as heglanced towards me. “I won’t conceal from you， Mr. Holmes，that we think in the C.I.D. that you have a wee bit of a bee inyour bonnet over this professor. I made some inquiries myselfabout the matter. He seems to be a very respectable， learned， andtalented sort of man.”
“I’m glad you’ve got so far as to recognize the talent.”
“Man， you can’t but recognize it！ After I heard your view I madeit my business to see him. I had a chat with him on eclipses. Howthe talk got that way I canna think； but he had out a reflectorlantern and a globe， and made it all clear in a minute. He lent mea book； but I don’t mind saying that it was a bit above my head，though I had a good Aberdeen upbringing. He’d have made agrand meenister with his thin face and gray hair and solemn-likeway of talking. When he put his hand on my shoulder as we wereparting， it was like a father’s blessing before you go out into thecold， cruel world.”
Holmes chuckled and rubbed his hands. “Great！” he said. “Great！
Tell me， Friend MacDonald， this pleasing and touching interviewwas， I suppose， in the professor’s study？”
“That’s so.”
“A fine room， is it not？”
“Very fine—very handsome indeed， Mr. Holmes.”
“You sat in front of his writing desk？”
“Just so.”
“Sun in your eyes and his face in the shadow？”
“Well， it was evening； but I mind that the lamp was turned onmy face.”
“It would be. Did you happen to observe a picture over theprofessor’s head？”
“I don’t miss much， Mr. Holmes. Maybe I learned that fromyou. Yes， I saw the picture—a young woman with her head on herhands， peeping at you sideways.”
“That painting was by Jean Baptiste Greuze.”
The inspector endeavoured to look interested.
“Jean Baptiste Greuze，” Holmes continued， joining his fingertips and leaning well back in his chair， “was a French artist whoflourished between the years 1750 and 1800. I allude， of course tohis working career. Modern criticism has more than indorsed thehigh opinion formed of him by his contemporaries.”
The inspector’s eyes grew abstracted. “Hadn’t we better—” hesaid.
“We are doing so，” Holmes interrupted. “All that I am sayinghas a very direct and vital bearing upon what you have called theBirlstone Mystery. In fact， it may in a sense be called the verycentre of it.”
MacDonald smiled feebly， and looked appealingly to me. “Yourthoughts move a bit too quick for me， Mr. Holmes. You leave outa link or two， and I can’t get over the gap. What in the whole wideworld can be the connection between this dead painting man andthe affair at Birlstone？”
“All knowledge comes useful to the detective，” remarkedHolmes. “Even the trivial fact that in the year 1865 a picture byGreuze entitled La Jeune Fille a l’Agneau fetched one million twohundred thousand francs—more than forty thousand pounds—atthe Portalis sale may start a train of reflection in your mind.”
It was clear that it did. The inspector looked honestlyinterested.
“I may remind you，” Holmes continued， “that the professor’ssalary 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. “Talkaway， 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？” heasked.
“We’ve time yet，” said the inspector， glancing at his watch. “I’vea 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 beforehe came. Once—well， I can hardly tell about the once to an officialdetective. It was on the last occasion that I took the liberty ofrunning over his papers—with the most unexpected results.”
“You found something compromising？”
“Absolutely nothing. That was what amazed me. However， youhave now seen the point of the picture. It shows him to be a verywealthy man. How did he acquire wealth？ He is unmarried. Hisyounger brother is a station master in the west of England. Hischair 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 itin an illegal fashion？”