Say – can you see?

An alchemist’s cabinet could have been the resource for modern eyeglass manufacture. The lenses contain a wealth of trace elements to improve their optical properties, from lanthanum (a rare earth, most of it mined in China today) to improve the refractive index and reduce color dispersion, to silver chloride or silver bromide in photochromic lenses that darken when exposed to ultraviolet light (UV).

Cerium, titanium, neodymium, and manganese in glass all help absorb harmful UV rays. Coloring and the non-reflective surface on sunglasses may contain erbium, one of the four rare-earth elements named for Ytterby, Sweden.

As with most glass, the fundamental component is high-purity silica, SiO₂ – quartz, the most common mineral in the earth’s crust. Plastic lenses are polycarbonate petroleum derivatives. Such plastics may also form the frames and earpieces, but the nose pads are softer plastic, often rubber-like silicones. Silicones are complex organic-silica compounds typically synthesized from methyl chloride (usually derived ultimately from natural gas or petroleum, and salt (NaCl) to give the chlorine) in the presence of copper as a catalyst.

Metal frames, usually some alloy of steel, can include aluminum, magnesium, titanium, and other elements added for tensile strength, light weight, and other properties. Nickel or chromium coatings give varied appearances as well as corrosion protection. “Memory metal,” which springs back to its original shape after flexing, is one of several copper-zinc-aluminum-nickel or nickel-titanium alloys.

The inventor of the first eyeglasses is unknown, but early Renaissance Italy, a hotbed of innovation, is the likely location where some lucky resident wore the first spectacles. Old manuscripts suggest that convex lenses – barely more than frame-mounted magnifying glasses – began to correct farsightedness in the late 1200s; not until 1451 did Nicholas of Cusa, a German Catholic bishop, learn how to use concave lenses to combat nearsightedness. Benjamin Franklin invented bifocals in 1784, and George Airy’s astigmatic lenses provided correction for the last of the major eye focusing problems in 1825, a decade before Airy was appointed Britain’s Astronomer Royal.

George Airy (1801-1892) is better known (at least among geophysicists like me) for his determination of the earth’s mean density when he was only in his 20s. His pendulum experiments led to the Airy Hypothesis of isostasy, which says that mountain ranges must have root structures of lower density than the surrounding rock, proportional to their height.

Image: The 'Glasses Apostle' in the altarpiece of the church of Bad Wildungen, Germany. Painted by Conrad von Soest in 1403, the 'Glasses Apostle' is considered the oldest depiction of eyeglasses north of the Alps. Image in public domain via Wikipedia.