In an instant he was on guard, but he saw no one.

The room was quiet, and it was empty.

The door into the hall was locked as he had left it.

All was the same, except that on the dressing-table was the cushion bearing two diamond pins instead of three.

The robbery had been done, as one might say, under the nose of the greatest detective in the world.

“Well, this takes my breath away,” said Nick to himself.  “It’s the nerviest challenge that ever was sprung on me.”

CHAPTER III.

How Nick found the jewels.

It certainly looked like sheer recklessness for this thief, whoever he might be, to play his game on Nick almost at the very moment when the great detective appeared upon the scene.

Shrewd as Nick was, he had not expected this.  His first thought, as the reader knows, was that it was a bold challenge, the defiance of a nervy criminal who thought himself absolutely safe from detection.

But a moment’s reflection made this seem less probable.

Was it not more natural to suppose that this event proved that the detective was unknown to the thief?

Such being the case, Colonel Richmond, his nephew and Mrs. Pond were acquitted at the start.

It may seem ridiculous to suspect them, in any case, but so strange was the nature of this affair that Nick gave nobody the credit of certain innocence.

Colonel Richmond was certainly very nearly crazy on one point.  He might be so much of a lunatic as to commit these robberies from simple delusion.  Or he might wish to prove to his daughter that the diamonds were not rightfully hers.

Mrs. Pond might be pawning them for small extravagances which she was afraid to have known.

As to Horace Richmond, there was no motive which seemed plausible.  The value of the articles taken was so small as to make the game not worth while for a man in his position.

And it was perfectly certain that no professional thief or dishonest servant was doing the work.

If such a person had been in the game, he would not have taken one of those diamond pins; he would have taken all three.

It was impossible to lose sight of the fact that the Stevenses would be the real gainers, if this ghost business led Colonel Richmond to insist that his daughter should give up the jewels.

Mrs. Stevens and her daughter could not be doing the job personally, but they might have a secret agent among the servants, or more probably concealed in some secret recess of the strange old house.

Nick resolved to go to see Mrs. and Miss Stevens without delay.  He hoped to judge by their conduct whether they knew anything about the robberies.

These thoughts passed through his mind in a flash.

He quickly searched the room to be sure that the thief was not concealed in it, and then descended to the main hall.  The outer door was open, and Colonel Richmond and his daughter were standing on the steps.