Dating back to China’s Han dynasty (202 BCE–220 CE), artisans made bronze mirrors with a peculiar talent. Look into the polished front and you see your reflection. Shine bright light onto it and aim the reflected beam at a wall, and a pattern appears in the light.

The mirrors were called touguang jing (透光镜), or “light-penetrating mirrors.” The name describes what they seemed to do. In fact, minute variations across the polished surface send light in slightly different directions, revealing a design too subtle to see on the mirror itself.

The mirror shows your face. The wall reveals the maker’s ingenuity.