"No, no, no!" cried Mortimer. "That will never do! Such a man might push his lunacy to the length of destroying a Jewish relic, but why on earth should he nibble round every stone so carefully that he can only do four stones in a night? We must have a better solution than that, and we must find it for ourselves, for I do not think that our inspector is likely to help us. First of all, what do you think of Simpson, the porter?" "Have you any reason to suspect him?" "Only that he is the one person on the premises." "But why should he indulge in such wanton destruction? Nothing has been taken away. He has no motive." "Mania?" "No, I will swear to his sanity." "Have you any other theory?" "Well, yourself, for example. You are not a somnambulist, by any chance?" "Nothing of the sort, I assure you." "Then I give it up." "But I don't--and I have a plan by which we will make it all clear." "To visit Professor Andreas?" "No, we shall find our solution nearer than Scotland. I will tell you what we shall do. You know that skylight which overlooks the central hall? We will leave the electric lights in the hall, and we will keep watch in the lumber-room, you and I, and solve the mystery for ourselves. If our mysterious visitor is doing four stones at a time, he has four still to do, and there is every reason to think that he will return to-night and complete the job." "Excellent!" I cried. "We shall keep our own secret, and say nothing either to the police or to Simpson. Will you join me?" "With the utmost pleasure," said I, and so it was agreed. It was ten o'clock that night when I returned to the Belmore Street Museum. Mortimer was, as I could see, in a state of suppressed nervous excitement, but it was still too early to begin our vigil, so we remained for an hour or so in his chambers, discussing all the possibilities of the singular business which we had met to solve. At last the roaring stream of hansom cabs and the rush of hurrying feet became lower and more intermittent as the pleasure-seekers passed on their way to their stations or their homes. It was nearly twelve when Mortimer led the way to the lumber-room which overlooked the central hall of the museum. He had visited it during the day, and had spread some sacking so that we could lie at our ease, and look straight down into the museum. The skylight was of unfrosted glass, but was so covered with dust that it would be impossible for anyone looking up from below to detect that he was overlooked. We cleared a small piece at each corner, which gave us a complete view of the room beneath us. In the cold, white light of the electric lamps everything stood out hard and clear, and I could see the smallest detail of the contents of the various cases.