Dr. Whiles rose slowly to his feet.

“You know the secret of that murder!” he declared.

“Why ask me?” the Prince answered.  “If I tell you that I do, you may find conscientious scruples about remaining here.  A man is not bound, you know, to give himself away.  Make the best of things, and do not try to see too far.”

The doctor was looking a little shaken.

“If you were mixed up in that affair,” he said, “and if I remain here when my evidence is needed, I become an accomplice.”

“Only if you remain here voluntarily,” the Prince reminded him cheerfully.  “Remember that and be comforted.  No effort that you could make now would bring you into touch with Mr. Inspector Jacks until I am quite prepared.  So you see, my dear doctor, that you have nothing with which to reproach yourself.  I will not insult you,” he continued, “by suggesting that a reward of fifty pounds could possibly have influenced your attitude.  If you have suffered your mind to dwell upon it for a single moment, try and remember the relative unimportance of such an amount when compared with a thousand guineas.”

The doctor moved to the window and back again.

“Supposing,” he said, “I decline to remain here?  Supposing I say that, believing you now to have a guilty knowledge of this murder, I repudiate our bargain?  Supposing I say that I will have nothing more to do with your thousand guineas,—­that I will leave this house?”

“Then we come to close quarters,” the Prince answered, “and you force me to tell you in plain words that, until I am ready for you to leave it, you are as much a prisoner in this room as though the keys of the strongest fortress in Europe were turned upon you.  I have told you this before.  I thought that we perfectly understood one another.”

“I did not understand,” the doctor protested.  “I knew that there was trouble, but I did not know that it was this!”

“The fact of your knowing or not knowing makes no difference,” the Prince answered.  “You are no longer a free agent.  The only question for you to decide is whether you remain here willingly or whether you will force me to remind you of our bargain.”

The doctor was sitting down again now.  All the time he watched the Prince with a gleam in his eyes, partly of horror, partly of fear.  He no longer doubted but that he was in the presence of a criminal.

“I am sorry,” the Prince continued, “that you have allowed this little matter to disturb you.  I thought that we had arranged it all at our last interview.  If you did not surmise my reasons for keeping you here, then I am afraid I gave you credit for more intelligence than you possess.  You will excuse me now, I am sure,” he added, rising.  “I have some letters to send off before I change.  By the bye, do you care to give me your parole?  It might, perhaps, lessen the inconvenience to which you are unfortunately subject.”