Chinese magic mirror

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 stresses set up by these processes caused the thinner parts of the surface to bulge outwards and become more convex than the thicker portions.

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.

[2][4] 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 family heirlooms.

[2][4] 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.

[2][4] 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 the British scientist William Bragg in 1932.