“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.” “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 completely obliterated. Finally, he polishes the face with rouge on a soft buff.