The lens that saved the ships

0:00 Before eighteen twenty-three,

0:02 a lighthouse was mostly a fire behind a

0:04 mirror,

0:05 and its light died within a few miles.

0:08 Augustin-Jean Fresnel worked out that the

0:11 useful part of a lens is only its surface

0:14 curvature,

0:15 so the bulk of the glass could be cut away

0:18 and the remainder arranged in concentric

0:21 rings.

0:21 His lens stood taller than a person,

0:24 weighed a fraction of what a solid lens

0:27 would,

0:27 and threw a beam more than twenty miles out

0:30 to sea.

0:31 Sailors called them beehives.

0:34 The same geometry now sits,

0:36 moulded in cheap plastic,

0:38 in overhead projectors,

0:40 car headlamps and the little magnifier on a

0:43 bookmark.