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The book continues from the previous article. We briefly described the development of ancient Chinese mirrors in the previous articles, and it is not difficult to find that each era has the aesthetic characteristics of mirror jewelry in each era. So we must not only ask: Why did the ancient Chinese in the Han and Tang Dynasties like to decorate the back of the mirror that could not look at people so complicated?
▲ [Tang] Sea beast grape pattern copper mirror
▲ [Tang] Flat screw back octagonal mirror
Collection of Shokura-in Temple, Nara, Japan
It turns out that the ancients did not have to turn the bronze mirror to enjoy these beautiful mirror back ornaments. In the middle and late Western Han Dynasty, there is a magical copper mirror, when the mirror reflects light on the wall, the pattern and inscription on the back of the mirror will appear in the light spot on the wall, as if there is a ray of light from the back of the mirror, and the pattern text on the back of the mirror is reflected on the wall through the copper mirror, so it is called "lens". The Tang legend "Ancient Mirror" tells the story of a lens that can illuminate the demon, and the author Wang Du's description of this ancient mirror is as follows: "If you inherit the sun, you will carry a painting on your back, ink into the shadow, and there is no loss." ”
▲ Shanghai Museum collection of Western Han Dynasty lenses
The Northern Song Dynasty scientist Shen Kuo also talked about this lens in his "Mengxi Pen Talk" and made some speculations about its transmission principle:
"There is a translucent light in the world, and there is an inscription on the back of the book, and where there are two crosses, the words are extremely ancient, and they cannot be read." In order to recognize the daylight, the back text and the two crosses are transparent, and the walls of the house are clearly distinguished. People have their own reasoning, to say that when casting, the thin place is cold first, but the difference in thickness on the back text is cold, and the copper shrinks more. Although the text is on the back, the face is hidden, so it appears in the light. From the perspective of the past, it is true. However, the Yu family has three books, and when he sees what his family has hidden, they are all the same, and the inscriptions on the text and painting are not different, and the shape is very ancient. Only this one is as transparent as this, and no one else can penetrate even if it is even thin. The Yigu people have their own techniques. ”
We translate this text into modern vernacular: "There is a kind of lens in the world, and there is an inscription on the back of the mirror, a total of two crosses, and the font is so ancient that no one can interpret it." With this mirror facing the sun, the pattern and twenty words on the back will be clearly projected on the wall of the room. Some people deduce the principle, believing that it is due to the first cooling of the thin part when casting, except for the thicker places with patterns and inscriptions, which cools slowly, resulting in copper shrinking more. Although the ornament is on the back, there are faint marks on the front of the mirror, so they appear in the light. I looked at the mirror and thought it was. However, my family has three mirrors, and I have seen the mirrors collected by other people's homes, all of which are of the same style, and the pattern inscriptions have no differences in the slightest, and the shape is very old. But only this kind of mirror can transmit light, and although other mirrors are also extremely thin, they cannot transmit light. I think the ancients had their own special production methods. ”
▲ Lens excavated from the Han Tomb in Dongdongshan, Xuzhou
The method of making the lens has been lost in the Song Dynasty, so Shen Kuo cannot fully explain the transmission principle of the "magic mirror", but his reasoning is still based. The bronze mirror is indeed very thick at the inscription and pattern, and thin where there is no inscription. Uneven thickness will cause casting stress on the copper mirror, and elastic deformation will occur when grinding the mirror, so there will be a difference in curvature. Even if this difference is too small to be detected by the naked eye, when the light is illuminated on the mirror surface, the curvature of the thick place is large, the reflected light is relatively dim, the projection is dark, the curvature of the thin place is small, and the light is concentrated, and the projection is relatively bright. In this way, the pattern on the back of the copper mirror can be reflected on the wall, and from the surface, it seems as if it can really "transmit light".
▲ [Western Han Dynasty] daylight symmetrical single-layer grass leaf pattern copper mirror, diameter 13.4 cm
Collection of Anji County Museum, Huzhou City, Zhejiang Province
"See the light of the sun, the world is bright" - the inscription of this Western Han dynasty lens embodies the ancients' infinite vision of the bright world, although this ideal country can only occasionally appear its ghost in the black technology that is constantly lost and restored in later generations.