The Chinese magic mirror is an ancient art that can be traced back to the Chinese Han dynasty. The mirrors were made out of solid bronze. The front is a shiny polished surface and could be used as a mirror, while the back has a design cast in the bronze. When bright sunlight or other bright light reflects onto the mirror, the mirror seems to become transparent. If that light is reflected from the mirror towards a wall, the pattern on the back of the mirror is then projected onto the wall. In about 800 AD, during the Tang dynasty, a book entitledRecord of Ancient Mirrors described the method of crafting solid bronze mirrors with decorations, written characters, or patterns on the reverse side that could cast these in a reflection on a nearby surface as light struck the front, polished side of the mirror; due to this seemingly transparent effect, they were called "light-penetration mirrors" by the Chinese. This Tang era book was lost over the centuries, but magic mirrors were described in the Dream Pool Essays by Shen Kuo, who owned three of them as a family heirloom. Perplexed as to how solid metal could be transparent, Shen guessed that some sort of quenching technique was used to produce tiny wrinkles on the face of the mirror too small to be observed by the eye. Although his explanation of different cooling rates was incorrect, he was right to suggest the surface contained minute variations which the naked eye could not detect; these mirrors also had no transparent quality at all, as discovered by William Bragg in 1932. Robert Temple describes their construction: "The basic mirror shape, with the design on the back, was cast flat, and the convexity of the surface produced afterwards by elaborate scraping and scratching. The surface was then polished to become shiny. The stresses set up by these processes caused the thinner parts of the surface to bulge outwards and become more convex than the thicker portions. Finally, a mercury amalgam was laid over the surface; this created further stresses and preferential buckling. The result was that imperfections of the mirror surface matched the patterns on the back, although they were too minute to be seen by the eye. But when the mirror reflected bright sunlight against a wall, with the resultant magnification of the whole image, the effect was to reproduce the patterns as if they were passing through the solid bronze by way of light beams." Michael Berry has written a paper describing the optics and giving some photos.
