A door opened out of the passage which was the private entrance from his rooms into the museum.  This he opened softly with his key, having first kicked off his shoes, an example which I followed.  We crept together through room after room, until the large hall lay before us, with that dark figure still stooping and working at the central case.  With an advance as cautious as his own we closed in upon him, but softly as we went we could not take him entirely unawares.  We were still a dozen yards from him when he looked round with a start, and uttering a husky cry of terror, ran frantically down the museum.

“Simpson!  Simpson!” roared Mortimer, and far away down the vista of electric lighted doors we saw the stiff figure of the old soldier suddenly appear.  Professor Andreas saw him also, and stopped running, with a gesture of despair.  At the same instant we each laid a hand upon his shoulder.

“Yes, yes, gentlemen,” he panted, “I will come with you.  To your room, Mr. Ward Mortimer, if you please!  I feel that I owe you an explanation.”

My companion’s indignation was so great that I could see that he dared not trust himself to reply.  We walked on each side of the old Professor, the astonished commissionaire bringing up the rear.  When we reached the violated case, Mortimer stopped and examined the breastplate.  Already one of the stones of the lower row had had its setting turned back in the same manner as the others.  My friend held it up and glanced furiously at his prisoner.

“How could you!” he cried.  “How could you!”

“It is horrible—­horrible!” said the Professor.  “I don’t wonder at your feelings.  Take me to your room.”

“But this shall not be left exposed!” cried Mortimer.  He picked the breastplate up and carried it tenderly in his hand, while I walked beside the Professor, like a policeman with a malefactor.  We passed into Mortimer’s chambers, leaving the amazed old soldier to understand matters as best he could.  The Professor sat down in Mortimer’s arm-chair, and turned so ghastly a colour that for the instant all our resentment was changed to concern.  A stiff glass of brandy brought the life back to him once more.

“There, I am better now!” said he.  “These last few days have been too much for me.  I am convinced that I could not stand it any longer.  It is a nightmare—­a horrible nightmare—­that I should be arrested as a burglar in what has been for so long my own museum.  And yet I cannot blame you.  You could not have done otherwise.  My hope always was that I should get it all over before I was detected.  This would have been my last night’s work.”

“How did you get in?” asked Mortimer.

“By taking a very great liberty with your private door.  But the object justified it.  The object justified everything.  You will not be angry when you know everything—­at least, you will not be angry with me.  I had a key to your side door and also to the museum door.  I did not give them up when I left.  And so you see it was not difficult for me to let myself into the museum.  I used to come in early before the crowd had cleared from the street.  Then I hid myself in the mummy-case, and took refuge there whenever Simpson came round.  I could always hear him coming.  I used to leave in the same way as I came.”