“Mr. Francis Howard went on to see Mr. Ferdinand Knopf.  This gentleman was having his warm bath, preparatory to going to bed.  So Robertson told the detective.  However, Mr. Knopf insisted on talking to Mr. Howard through his bath-room door.  Mr. Knopf thanked him for all the trouble he was taking, and felt sure that he and Mr. Shipman would soon recover possession of their diamonds, thanks to the persevering detective.

“He! he! he!” laughed the man in the corner.  “Poor Mr. Howard.  He persevered—­but got no farther; no, nor anyone else, for that matter.  Even I might not be able to convict the thieves if I told all I knew to the police.

“Now, follow my reasoning, point by point,” he added eagerly.

“Who knew of the presence of the diamonds in the house of Mr. Shipman and Mr. Knopf?  Firstly,” he said, putting up an ugly claw-like finger, “Mr. Shipman, then Mr. Knopf, then, presumably, the man Robertson.”

“And the tramp?” said Polly.

“Leave the tramp alone for the present since he has vanished, and take point number two.  Mr. Shipman was drugged.  That was pretty obvious; no man under ordinary circumstances would, without waking, have his keys abstracted and then replaced at his own bedside.  Mr. Howard suggested that the thief was armed with some anaesthetic; but how did the thief get into Mr. Shipman’s room without waking him from his natural sleep?  Is it not simpler to suppose that the thief had taken the precaution to drug the jeweller before the latter went to bed?”

“But—­”

“Wait a moment, and take point number three.  Though there was every proof that Mr. Shipman had been in possession of L25,000 worth of goods since Mr. Knopf had a cheque from him for that amount, there was no proof that in Mr. Knopf’s house there was even an odd stone worth a sovereign.

“And then again,” went on the scarecrow, getting more and more excited, “did it ever strike you, or anybody else, that at no time, while the tramp was in custody, while all that searching examination was being gone on with, no one ever saw Mr. Knopf and his man Robertson together at the same time?

“Ah!” he continued, whilst suddenly the young girl seemed to see the whole thing as in a vision, “they did not forget a single detail—­follow them with me, point by point.  Two cunning scoundrels—­geniuses they should be called—­well provided with some ill-gotten funds—­but determined on a grand coup.  They play at respectability, for six months, say.  One is the master, the other the servant; they take a house in the same street as their intended victim, make friends with him, accomplish one or two creditable but very small business transactions, always drawing on the reserve funds, which might even have amounted to a few hundreds—­and a bit of credit.

“Then the Brazilian diamonds, and the Parisians—­which, remember, were so perfect that they required chemical testing to be detected.  The Parisian stones are sold—­not in business, of course—­in the evening, after dinner and a good deal of wine.  Mr. Knopf’s Brazilians were beautiful; perfect!  Mr. Knopf was a well-known diamond merchant.