“That is a perfectly sound explanation,” said Thorndyke. “But I think there is something more. You will notice that the label is an old one; it must have been on some years, to judge by its dingy appearance, whereas the two mirror-plates look to me comparatively new. But we can soon put that matter to the test, for the label was evidently stuck on when the frame was new, and if the plates were screwed on at the same time, the wood that they cover will be clean and new-looking.”

He drew from his pocket a “combination” knife containing, among other implements, a screwdriver, with which he carefully extracted the screws from one of the little brass plates by which the frame had been suspended from the nails.

“You see,” he said, when he had removed the plate and carried the photograph over to the gasjet, “the wood covered by the plate is as dirty and time-stained as the rest of the frame. The plates have been put on recently.”

“And what are we to infer from that?”