The Last Codex
EN

Theme

Language

Eyeglasses

Lenses that extend a working life

Date
c. 1286
Era
Middle Ages
Rebuild stage
III Crafts & cities
Difficulty

Plate

Legend
  1. 1Biconvex glass lens
  2. 2Bone rim
  3. 3Handle
  4. 4Iron rivet
  5. 5Copper wire binding

Anatomy

×10 · ⌀ 0.14 mm
Light from one point of the pageEye and spectacle lens: the half behind the cutCut faces, the sclera and the bone rim hatchedThe eye and the spectacle lens are cut through the middle at true scale, and the light from one point of a page 40 cm away is traced through both. The eye is the Gullstrand-Emsley schematic eye, 23.9 mm long, relaxed and with no focusing left, as in most people from their mid-fifties. On its own it brings the page to a focus 0.95 mm behind the retina, so the light arrives as a patch 0.14 mm wide; the faint lines show where it would meet. The circle shows the back of the eye ten times larger: where the rays cross, and the patch of light they leave on the retina. Light from a page that close is still spreading when it reaches the eye. A convex lens takes away part of that spread before the cornea, and at about +2.6 D the light leaves it parallel, as if from a distant object, so the focus falls on the retina. The readout calls the image sharp from +2.50 to +2.75 D, where the error stays within the eye's depth of focus of about ±0.3 D. At +3.5 D the focus lies 0.33 mm in front of the retina and the page blurs again unless it is brought closer. The lens sits 14 mm in front of the cornea, and the patch sizes assume a pupil 3 mm across.

Eyeglasses are a pair of lenses held in a frame in front of the eyes to correct long sight, short sight or astigmatism. They appeared in Italy in the 1280s as reading aids for older people; lenses for short sight followed in the fifteenth century and side arms in the eighteenth.

What it is

Eyeglasses, or spectacles, put a lens in front of each eye so that the light reaches the retina already bent by the right amount. The first ones helped people in their forties and older to read and to do fine work, once focusing at reading distance had become hard.

The pair on the plate is the oldest type, the rivet spectacles of the fourteenth and fifteenth centuries. Each glass lens sits in a ring carved from bone, wood or horn. A short handle sticks out from each ring, and the two handles are joined by a single rivet. The pair has no side arms: the rivet rests on the bridge of the nose and the rims pinch the nose on either side, so the wearer has to keep the head level or hold the spectacles with one hand.

How it works

A clear dome of rock crystal resting on a dried leaf, enlarging the veins beneath it A reading stone, a dome of crystal or glass laid on the page, was the magnifier that came before spectacles.

Inside the eye, a soft lens changes shape to bring near or distant things into focus. With age it hardens, and the closest point the eye can see sharply moves away: a few centimetres from a child’s face, it ends up out of reach of the arms for most people of sixty. This is presbyopia, and it is why reading glasses were the first ones made.

A convex lens, thicker in the middle than at the edge, makes the light converge a little before it enters the eye, and so does part of the work that the eye’s own lens can no longer do. Opticians measure this power in dioptres, the inverse of the focal length in metres. A reading lens of +2 dioptres brings parallel light to a point 50 cm behind it.

Short sight is a different focusing problem, and it can come on top of presbyopia. The eyeball is a little too long, or its optics too strong, and distant things blur. A concave lens, thinner in the middle, spreads the light slightly before it enters the eye and pushes the image back onto the retina. Lenses of this kind were being ground in Florence in the fifteenth century. In astigmatism the eye’s power differs between two perpendicular directions, so lines at one angle blur more than lines at another. Correcting it needs a lens curved more in one direction than the other, and such lenses came much later.

How to build it

A pair of rivet spectacles like the one on the plate takes two lenses, a piece of bone or hardwood, copper wire and one small rivet.

A bronze grinding tool on a wooden block, a glass blank fixed with pitch to a wooden handle, a pot of abrasive and a piece of leather The glass blank, fixed with pitch to a wooden handle, was worked with wet abrasive in a bronze tool, then polished on leather.

Get the lenses. For a pair you will actually look through, take two lenses from a cheap pair of modern reading glasses. Grinding your own is a historical demonstration: as a spectacle maker did, rub a disc of glass about 4 cm across with wet abrasive powder inside a tool hollowed to the curve you want, finishing with finer and finer grits, then polish it with rouge on leather or felt. For a +2 dioptre lens made of ordinary glass, both faces need a radius of curvature of about 50 cm, a very shallow curve. Hand-ground lenses will not have an exact power, and their optical centres will not be lined up with your pupils.

Cut the rims. Mark two circles on a flat piece of bone or boxwood with dividers, a little larger than the lenses, and saw them out with a fine saw, leaving a short tab on each ring for the handle. Cut a V-shaped groove around the inside of each rim to hold the edge of the lens.

Split and bind. Saw through each rim on the side opposite the handle. Spring the ring open, slide the lens into the groove and close the split by wrapping thin copper wire several times around two small lobes left on either side of it.

Rivet the handles. Drill a small hole near the end of each handle, overlap them and set a copper or iron rivet, peened just tight enough that the two halves can still be folded. File small notches along the inner edges where the rims meet the nose, so they grip better.

Safety. Grind glass wet and wear a dust mask: dry glass and abrasive dust harm the lungs. Wear eye protection when drilling bone and cutting wire, and handle cut glass with care. Never look at the Sun through a lens, and never leave one in sunlight near paper or wood, because it can start a fire. This pair is a replica: do not use it in place of glasses prescribed and fitted by an optometrist, and never drive with it.

History and evolution

Polished crystals and glass had been used to enlarge writing long before spectacles. The step of setting a lens in a frame in front of each eye came in Italy around 1280 to 1290. Where exactly is not known: Pisa and Florence are often proposed, and a Venetian ordinance of 1300 already punished glassmakers who sold glass “discs for the eyes” as rock crystal. In a sermon given in Florence in 1306, the Dominican friar Giordano da Pisa said that the art of making spectacles was not yet twenty years old and that he had spoken with the man who first made them. Nobody knows his name.

The oldest known painting of spectacles is a fresco of 1352 by Tommaso da Modena in the chapter house of San Nicolò in Treviso, which shows a Dominican cardinal reading with rivet spectacles. Real frames rarely survive. A bone pair from the fifteenth century was found at Trig Lane, by the Thames in London, at a time when thousands of spectacles a year were imported into London from the Low Countries. In 1466 the Duke of Milan ordered two hundred pairs graded by age, a sign that lens strengths had become fairly standard.

Late eighteenth-century steel spectacles with side arms ending in rings, each lens divided in two halves along a horizontal line Double spectacles of the late eighteenth century: a distance lens above, a reading lens below.

By the early sixteenth century spectacles could correct short sight as well as long sight, and bridges of leather or whalebone replaced the rivet. Around 1730 the London optician Edward Scarlett is credited with adding rigid side arms, or temples, that pressed against the sides of the head. In letters of 1784 and 1785 Benjamin Franklin described his “double spectacles”, with half of each lens for distance and half for reading. Pince-nez, held by a spring on the nose, followed in the nineteenth century, then in the twentieth plastic frames, lighter plastic lenses and, later, thinner high-index lenses.

Type a name, a date, a material, a domain…