The ancients cast mirrors.
When the ancients made mirrors, the big mirror was flat and the small mirror was convex. Concave mirror reflects a larger face image, while convex mirror reflects a smaller face image. So make it slightly convex to make the face smaller, so that even if the mirror is small, it can still capture the whole face ... This craft is ingenious, and later people can't do it. Once you get the ancient mirror, you should scrape it smooth. This is why Shi Kuang (a famous musician in the Spring and Autumn Period) is very sad, and no one really knows the melody!