Chinese magic mirror

The Chinese magic mirror () is an ancient art that can be traced back to the Chinese Han dynasty (206 BC – 24 AD). 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 (618–907), a book entitled "Record 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 (1031–1095), 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 (after an entire century of their confounding Western scientists).
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.
History.
China.
The ancient craftsmanship of Chinese magic mirrors dates back to 2900 - 2000 BC. in China, Egypt and the Indus Valley. These bronze mirrors became popular and were produced in large quantities during the Han dynasty between the period of 206 BC. and 24 AD. mainly in China. According to UNESCO, around 800 AD, during the Tang Dynasty (618-907), the book "Register of Ancient Mirrors" described how to make those mirrors which the Chinese called "translucent mirrors". Although it eventually got lost, Shen Kuo (1031-1095), who owned three mirrors as a family relic, described them in his Essay on the "Treasure of Dreams". Surprised by the ability of a solid metal to behave as if it were transparent, Shen assumed that in its creation some tempering technique was performed to produce surface wrinkles imperceptible to the human eye. Although he was wrong about cooling rates, he was right about the cause that left 19th century Western scientists unanswered. And it was not until 1932 that William Bragg discovered that Shen was right about imperceptible surface wrinkles.
Japan.
On the other hand, as the manufacture of mirrors in China increased, it expanded to Korea and Japan. In fact, Emperor Cao Rui and the Wei Kingdom of China gave numerous bronze mirrors (known as "Shinju-kyo" in Japan) to Queen Himiko of Wa (Japan), where they were received as rare and mysterious objects. They were described as "sources of honesty" as they were said to reflect all good and evil without error. That is why Japan considers to be one of the three great imperial treasures a sacred mirror called Yata-no-Kagami.
Today, Yamamoto Akihisa is said to be the last manufacturer of magic mirrors in Japan. The "Kyoto Journal" interviewed the craftsman and he explained a small portion of the technique, that he learned from his father.
Western Europe.
For many centuries, the "magic" of these mirrors baffled both lay people and scientists, who devoted themselves to do different research work on this subject.
The first magic mirror to appear in Western Europe was owned by the director of the Paris Observatory, who, on his return from China, brought several mirrors and one of them was magical. The latter was presented as an irresistible unknown object to the French Academy of Sciences in 1844. No one had seen anything like it, and no matter how much they studied its behavior, they could never fully understand it. In total, there were only four magic mirrors brought from China to Europe; but in 1878, two engineering professors presented to the Royal Society of London several models they had brought from Japan. The English called the artefacts "open mirrors" and for the first time made technical observations regarding their construction. The mirrors effects were so wonderful that the Royal Society was mesmerized by them. No one, however, could figure out what produced the spooky and beautiful projection of light which they categorized as an "impossible optical illusion" and therefore "magical".
Later, in November 2005, the physicist Michael Berry, made a project on this topic and published an article describing the optics.