Inside the Glass Factory, Xia Huai'an was instructing several grinding workers on how to grind spectacle lenses.
Holding two circular glass blanks, Xia Huai'an demonstrated to the workers, "The principle of the reducing mirror lies in the refraction of light—the lens surface is not flat, but forms a slightly concave arc."
He traced an imaginary parabola on the glass surface with his fingertip. "This kind of curved surface makes light refract inward, making the human reflection appear slender and thin. If the curve is even larger, it can make a person look like a thin bamboo pole. Conversely, a slightly convex arc can make someone look like a big fat man."
Chapter 107