“They must have their spy somewhere,” said Mr. Bruff, looking at the dark sailor in his turn.  “And he may be the man.”

Before he could say more, his coat-tail was respectfully pulled by his attendant sprite with the gooseberry eyes.  Mr. Bruff looked where the boy was looking.  “Hush!” he said.  “Here is Mr. Luker!”

The money-lender came out from the inner regions of the bank, followed by his two guardian policemen in plain clothes.

“Keep your eye on him,” whispered Mr. Bruff.  “If he passes the Diamond to anybody, he will pass it here.”

Without noticing either of us, Mr. Luker slowly made his way to the door—­now in the thickest, now in the thinnest part of the crowd.  I distinctly saw his hand move, as he passed a short, stout man, respectably dressed in a suit of sober grey.  The man started a little, and looked after him.  Mr. Luker moved on slowly through the crowd.  At the door his guard placed themselves on either side of him.  They were all three followed by one of Mr. Bruff’s men—­and I saw them no more.

I looked round at the lawyer, and then looked significantly towards the man in the suit of sober grey.  “Yes!” whispered Mr. Bruff, “I saw it too!” He turned about, in search of his second man.  The second man was nowhere to be seen.  He looked behind him for his attendant sprite.  Gooseberry had disappeared.

“What the devil does it mean?” said Mr. Bruff angrily.  “They have both left us at the very time when we want them most.”

It came to the turn of the man in the grey suit to transact his business at the counter.  He paid in a cheque—­received a receipt for it—­and turned to go out.

“What is to be done?” asked Mr. Bruff.  “We can’t degrade ourselves by following him.”

“I can!” I said.  “I wouldn’t lose sight of that man for ten thousand pounds!”

“In that case,” rejoined Mr. Bruff, “I wouldn’t lose sight of you, for twice the money.  A nice occupation for a man in my position,” he muttered to himself, as we followed the stranger out of the bank.  “For Heaven’s sake don’t mention it.  I should be ruined if it was known.”

The man in the grey suit got into an omnibus, going westward.  We got in after him.  There were latent reserves of youth still left in Mr. Bruff.  I assert it positively—­when he took his seat in the omnibus, he blushed!

The man in the grey suit stopped the omnibus, and got out in Oxford Street.  We followed him again.  He went into a chemist’s shop.

Mr. Bruff started.  “My chemist!” he exclaimed.  “I am afraid we have made a mistake.”

We entered the shop.  Mr. Bruff and the proprietor exchanged a few words in private.  The lawyer joined me again, with a very crestfallen face.

“It’s greatly to our credit,” he said, as he took my arm, and led me out—­“that’s one comfort!”

“What is to our credit?” I asked.

“Mr. Blake! you and I are the two worst amateur detectives that ever tried their hands at the trade.  The man in the grey suit has been thirty years in the chemist’s service.  He was sent to the bank to pay money to his master’s account—­and he knows no more of the Moonstone than the babe unborn.”