copper and its alloys. If a plate of such metal or alloy—say, bronze, for instance—is made red-hot and quenched in water, it becomes quite soft and flexible—the reverse of what happens in the case of iron. Now, if such a plate of softened metal be placed on a steel anvil and hammered, it becomes extremely hard and brittle.”
“I follow that,” said Brodribb.
“Then see what follows. If, instead of hammering the soft plate, you put on it the edge of a blunt chisel and strike on that chisel a sharp blow, you produce an indented line. Now the plate remains soft; but the metal forming the indented line has been hammered and has become hard. There is now a line of hard metal on the soft plate. Is that clear?”
“Perfectly,” replied Brodribb; and Thorndyke accordingly continued: “The second fact is this: If a beam of light falls on a polished surface which reflects it, and if that surface is turned through a given angle, the beam of light is deflected through double that angle.”
“H’m!” grunted Brodribb. “Yes. No doubt. I hope we are not going to get into any deeper waters, Thorndyke.”
“We are not,” replied the latter, smiling urbanely. “We are now going to consider the application of these facts. Have you ever seen a Japanese magic mirror?
“Never; nor even heard of such a thing.”
“They are bronze mirrors, just like the ancient Greek or Etruscan mirrors—which are probably ‘magic’ mirrors, too. A typical specimen consists of a circular or oval plate of bronze, highly polished on the face and decorated on the back with chased ornament—commonly a dragon or some such device—and furnished with a handle. The ornament is, as I have said, chased; that is to say, it is executed in indented lines made with chasing tools, which are, in effect, small chisels, more or less blunt, which are struck with a chasing-hammer.
“Now these mirrors have a very singular property. Although the face is perfectly plain, as a mirror should be, yet, if a beam of sunlight is caught on it and reflected, say, on to a white wall, the round or oval patch of light on the wall is not a plain light patch. It shows quite clearly the ornament on the back of the mirror.”
“But how extraordinary!” exclaimed Miss Bonney.
“It sounds quite incredible.” I said.
“It does,” Thorndyke agreed. “And yet the explanation is quite simple. Professor Sylvanus Thompson pointed it out years ago. It is based on the facts which I have just stated to you. The artist who makes one of these mirrors begins, naturally, by annealing the metal until it is quite soft. Then he chases the design on the back, and this design then shows slightly on the face. But he now grinds the face perfectly flat with fine emery and water so that the traces of the design are complete obliterated. Finally, he polishes the face with rouge on a soft buff.
“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 buffer 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 be 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.
“With these considerations in my mind, I examined the casket, and this was what I found. The thing, itself, was a common white-metal casting, made presentable by means of lacquer. But the white metal bottom had been cut out and replaced by a plate of fine bronze—Shakudo. The inside of this was covered with an etched design, which immediately aroused my suspicions. Turning it over, I saw that the outside of the bottom was not only smooth and polished; it was a true mirror. It gave a perfectly undistorted reflection of my face. At once, I suspected that the mirror held the secret; that the message, whatever it was, had been chased on the back, had then been scraped away and an etched design worked on it to hide the traces of the scraper.
“As soon as you were gone, I took the casket up to the laboratory and threw a strong beam of parallel light from a condenser on the bottom, catching the reflection on a sheet of white paper. The result was just what I had expected. On the bright oval patch on the paper could be seen the shadowy, but quite distinct, forms of five words in the Japanese character.
“I was in somewhat of a dilemma, for I have no knowledge of Japanese, whereas the circumstances were such as to make it rather unsafe to employ a translator. However, as I do just know the Japanese characters and possess a Japanese dictionary, I determined to make an attempt to fudge out the words myself. If I failed, I could then look for a discreet translator.
“However, it proved to be easier than I had expected, for the words were detached; they did not form a sentence, and so involved no questions of grammar. I spelt out the first word and then looked it up in the dictionary. The translation was ‘pearls.’ This looked hopeful, and I went on to the next, of which the translation was ‘pump.’ The third word floored me. It seemed to be ‘jokkis,’ or ‘jokkish,’ but there was no such word in the dictionary; so I turned to the next word, hoping that it would explain its predecessor. And it did. The fourth word was ‘fields,’ and the last word was evidently ‘London.’ So the entire group read ‘Pearls, Pump, Jokkis, Fields, London.’
“Now, there is no pump, so far as I know, in Jockey Fields, but there is one in Bedford Row close to the corner of the Fields, and exactly opposite the end of Brownlow Street And by Mr. Brodribb’s account, Uyenishi, in his flight, ran down Hand Court and re turned up Brownlow Street, as if he were making for the pump. As the latter is disused and the handle-hole is high up, well out of the way of children, it offers quite a good temporary hiding-place, and I had no doubt that the bag of pearls had been poked into it and was probably there still. I was tempted to go at once and explore; but I was anxious that the discovery should be made by Miss Bonney, herself, and I did not dare to make a preliminary exploration for fear of being shadowed. If I had found the treasure I should have had to take it and give it to her; which would have been a flat ending to the adventure.
