A lens bends light only at its surfaces, where the light goes into the glass and where it comes out. Everything in between just carries it across. For a reading glass that hardly matters, but a lens big enough to throw a lighthouse beam out to sea would be a slab so thick it cracked, sagged and swallowed the light it was meant to send. Augustin Fresnel’s answer, lit for the first time at the Cordouan lighthouse in France in 1823, was to keep the curves and throw away the middle: the lens cut into concentric rings of prism, each holding only the slice of surface that did the bending, stepped back flat. Buffon had sketched a one-piece version in the 1740s, and Fresnel said so in print. The glass that was taken out had not been doing anything wrong. It had been doing exactly what glass does, and that was the trouble.