“It looks to me,” said the police inspector, “as if someone had been trying to take out the stones.”

“My fear is,” said Mortimer, “that he not only tried, but succeeded.  I believe these four stones to be skilful imitations which have been put in the place of the originals.”

The same suspicion had evidently been in the mind of the expert, for he had been carefully examining the four stones with the aid of a lens.  He now submitted them to several tests, and finally turned cheerfully to Mortimer.

“I congratulate you, sir,” said he, heartily.  “I will pledge my reputation that all four of these stones are genuine, and of a most unusual degree of purity.”

The colour began to come back to my poor friend’s frightened face, and he drew a long breath of relief.

“Thank God!” he cried.  “Then what in the world did the thief want?”

“Probably he meant to take the stones, but was interrupted.”

“In that case one would expect him to take them out one at a time, but the setting of each of these has been loosened, and yet the stones are all here.”

“It is certainly most extraordinary,” said the inspector.  “I never remember a case like it.  Let us see the watchman.”

The commissionaire was called—­a soldierly, honest-faced man, who seemed as concerned as Ward Mortimer at the incident.

“No, sir, I never heard a sound,” he answered, in reply to the questions of the inspector.  “I made my rounds four times, as usual, but I saw nothing suspicious.  I’ve been in my position ten years, but nothing of the kind has ever occurred before.”

“No thief could have come through the windows?”

“Impossible, sir.”

“Or passed you at the door?”

“No, sir; I never left my post except when I walked my rounds.”

“What other openings are there in the museum?”

“There is the door into Mr. Ward Mortimer’s private rooms.”

“That is locked at night,” my friend explained, “and in order to reach it anyone from the street would have to open the outside door as well.”

“Your servants?”

“Their quarters are entirely separate.”

“Well, well,” said the inspector, “this is certainly very obscure.  However, there has been no harm done, according to Mr. Purvis.”

“I will swear that those stones are genuine.”

“So that the case appears to be merely one of malicious damage.  But none the less, I should be very glad to go carefully round the premises, and to see if we can find any trace to show us who your visitor may have been.”