“But now observe that wherever the chasing-tool has made a line, the metal is hardened right through, so that the design is in hard metal on a soft matrix. But the hardened metal resists the wear of the polishing buff more than the soft metal does. The result is that the act of polishing causes the design to appear in faint relief on the face. Its projection is infinitesimal--less than the hundred-thousandth of an inch--and totally invisible to the eye. But, minute as it is, owing to the optical law which I mentioned--which, in effect, doubles the projection--it is enough to influence the reflection of light. As a consequence, every chased line appears on the patch of light as a dark line with a bright border, and so the whole design is visible. I think that is quite clear.” “Perfectly clear,” Miss Bonney and Brodribb agreed. “But now,” pursued Thorndyke, “before we come to the casket, there is a very curious corollary which I must mention. Supposing our artist, having finished the mirror, should proceed with a scraper to erase the design from the back; and on the blank, scraped surface to etch a new design. The process of etching does not harden the metal, so the new design does not appear on the reflection. But the old design would. For although it was invisible on the face and had been erased from the back, it would still exist in the substance of the metal and continue to influence the reflection. The odd result would be that the design which would be visible in the patch of light on the wall would be a different one from that on the back of the mirror. “No doubt, you see what I am leading up to. But I will take the investigation of the casket as it actually occurred. It was obvious, at once, that the value of the thing was extrinsic. It had no intrinsic value, either in material or workmanship. What could that value be? The clear suggestion was that the casket was the vehicle of some secret message or information. It had been made by Uyenishi, who had almost certainly had possession of the missing pearls, and who had been so closely pursued that he never had an opportunity to communicate with his confederates. It was to be given to a man who was almost certainly one of those confederates; and, since the pearls had never been traced, there was a distinct probability that the (presumed) message referred to some hiding-place in which Uyenishi had concealed them during his flight, and where they were probably still hidden.