“Before my time,” said Billy. He drew the window curtains apart and looked out into the street. “There are folk who watch us from over yonder. I can see a fellow now at the window. Have a look for yourself.”

Watson had taken a step forward when the bedroom door opened, and the long, thin form of Holmes emerged, his face pale and drawn, but his step and bearing as active as ever. With a single spring he was at the window, and had drawn the blind once more.

“That will do, Billy,” said he. “You were in danger of your life then, my boy, and I can’t do without you just yet. Well, Watson, it is good to see you in your old quarters once again. You come at a critical moment.”

“So I gather.”

“You can go, Billy. That boy is a problem, Watson. How far am I justified in allowing him to be in danger?”

“Danger of what, Holmes?”

“Of sudden death. I’m expecting something this evening.”

“Expecting what?”

“To be murdered, Watson.”

“No, no, you are joking, Holmes!”

“Even my limited sense of humour could evolve a better joke than that. But we may be comfortable in the meantime, may we not? Is alcohol permitted? The gasogene and cigars are in the old place. Let me see you once more in the customary armchair. You have not, I hope, learned to despise my pipe and my lamentable tobacco? It has to take the place of food these days.”

“But why not eat?”

“Because the faculties become refined when you starve them. Why, surely, as a doctor, my dear Watson, you must admit that what your digestion gains in the way of blood supply is so much lost to the brain. I am a brain, Watson. The rest of me is a mere appendix. Therefore, it is the brain I must consider.”

“But this danger, Holmes?”

“Ah, yes, in case it should come off, it would perhaps be as well that you should burden your memory with the name and address of the murderer. You can give it to Scotland Yard, with my love and a parting blessing. Sylvius is the name - Count Negretto Sylvius. Write it down, man, write it down! 136 Moorside Gardens, N. W. Got it?”

Watson’s honest face was twitching with anxiety. He knew only too well the immense risks taken by Holmes and was well aware that what he said was more likely to be under-statement than exaggeration. Watson was always the man of action, and he rose to the occasion.

“Count me in, Holmes. I have nothing to do for a day or two.”

“Your morals don’t improve, Watson. You have added fibbing to your other vices. You bear every sign of the busy medical man, with calls on him every hour.”

“Not such important ones. But can’t you have this fellow arrested?”

“Yes, Watson, I could. That’s what worries him so.”

“But why don’t you?”

“Because I don’t know where the diamond is.”

“Ah! Billy told me - the missing Crown jewel!”

“Yes, the great yellow Mazarin stone. I’ve cast my net and I have my fish. But I have not got the stone. What is the use of taking them? We can make the world a better place by laying them by the heels. But that is not what I am out for. It’s the stone I want.”

“And is this Count Sylvius one of your fish?”

“Yes, and he’s a shark. He bites. The other is Sam Merton, the boxer. Not a bad fellow, Sam, but the Count has used him. Sam’s not a shark. He is a great big silly bull-headed gudgeon. But he is flopping about in my net all the same.”

“Where is this Count Sylvius?”