This is the first on a series of three posts about glasses.
Once upon a time, there was a young couple and their mischievous four year old kid, Robert. When his brother was born without ears, his dad strongly advised him to keep his teasing mouth shut to avoid hurting his mum’s feelings. “How good is his sight?” asked Robert when both his mum and brother came back from the hospital. “His sight?” responded his mum, intrigued. “Yes, his sight, because if he happens to need glasses, he’s screwed!”
Spectacles, commonly known as eyeglasses, were invented sometime between 1285 and 1290. Nevertheless, as the Italian physicist Vasco Ronchi put it, “when it is all summed up, the fact remains that this world has found lenses on its nose without knowing whom to thank.” But not only whom to thank remains a mystery, how one of the most important inventions of all time was possible at this early stage of our history has also remained a highly speculative question.
Common sense seems to indicate that all inventions are built upon a solid base of previous knowledge and experience. In the case of spectacles, though, that may not be the case—this is precisely the subject of in-depth twentieth century research conducted by Albert Van Helden, Jean-Paul Wayenborgh, and Rolf Willach.
In the second half of the thirteenth century, the scenario set for the invention of spectacles was far from being optimum. On the one hand, remedies for the weakness of old eyes (i.e., presbyopia) were sought from a medical standpoint rather than from an optical one and the optical principles of the eye were wholly unknown—they were firstly developed by Johannes Kepler in 1604. On the other hand, there was no technology available to manufacture quality glass, nor to precisely grind and polish lenses, much less in identical pairs.
Reading stones were widely used by monks in the High Middle Ages, and already known in antiquity. These plano-convex surfaces were able to more or less magnify the writing when reading it, but left the ophthalmological problem unsolved; the image in the retina for those with eye problems remained, albeit magnified, unclear. Thus, for writing, for example, the stones were entirely useless. Nevertheless, monks somehow figured out a way to use convex dioptric vision aids with small refracting powers to improve the vision of presbyopic eyes.
Before the First Crusade, in towns and monasteries across Europe, relics (e.g., body parts, clothes, or utensils of the apostles) were kept hidden in reliquaries, and used to seek the attention of pilgrims, whose presence naturally brought income to the region. The sole presence of the (hidden) relics in these reliquaries, most of which were hand-made by monks, was enough for the faithful. The Crusades and an ascending Christian pride meant one fundamental change to the design of reliquaries; relics were no longer to be hidden, but shown behind the protection of transparent stone-ground rock-crystal windows. It is fair to assume that the many older monks who suffered from presbyopia in monasteries shortly realized the optical properties of the best of such crystal plates, which were mounted in wooden frames, and started using them as dioptric vision aids—and thus the magnifying glass was born. The oldest known depiction of a magnifying glass is part of a sculpture in the cathedral of Constance, in the heart of Germany, which was built around 1250, about 100 years before the oldest known depiction of spectacles in a painting by Tomaso da Modena in 1352.
While magnifying glasses probably developed as early as the first half of the eleventh century, shortly after the First Crusade, spectacles had still several obstacles to overcome. For one, the stone-grinding of rock-crystals, which happened mostly in monasteries and stone-grinding workshops, was not only time-consuming and skill-requiring, but useless at producing pairs of nearly identical lenses. Furthermore, glass, the one alternative to rock-crystals was, at the time, completely useless for optical applications, and had not even been considered as such yet—it was not until the last quarter of the thirteenth century in Venice.
Four were the key ingredients of glassblowing (i.e., a vitreous substance, a melting-point reducer, a stabilizer, and additives). While the vitreous substance’s melting-point was already achievable in wood-fired glass-melting ovens back then, a melting-point reducer was used to reduce the melting temperature of the mixture. Low temperatures leaded to opaque air bubbles in the so-called white (i.e., uncolored) glass, while high temperatures introduced more or less color by unwanted pollution from different substances. Therefore, two things were needed to improve the quality of the so-called white glass, an efficient energy source and a mixture made of low impact pollutants. Serendipity brought those two advancements to Venice, a city without important wood resources, in which firebricks made of the best clay were used to heat glass-melting ovens. The right ingredients (i.e., mostly natron, a naturally occurring sodium-based mixture) reached the Italian city from Egypt after Venice became an emerging trading arena, once the Crusades ended. The combination of the right ingredients and the right energy source made of Venetian cristallum the best white glass ever produced, and the envy of Italian glassmakers everywhere.
What comes next is not known in detail, but it is definitely one of the first examples of interdisciplinary design thinking. The rumor of the invention of a pristine uncolored glass reached Dominican monasteries in which monks were stone-grinding rock-crystals to make magnifying glasses. It is fair to assume that one of those knowledgeable monks considered the possibility of using cristallum instead of rock-crystals to make dioptric vision aids and challenged Venetian glassworkers with the task. It was only then when one of the glassworkers may have been able to come up with an obvious (from a glassworker’s point of view), yet ingenious, idea. “You should not make the vision aid by grinding. You should, instead, blow it up as a glass ball of the required diameter. Then by cutting this ball into many small discs and grinding their concave surfaces plane, you will get a lot of equal plates, and because they all have the same shape, you could arrange them in pairs in front of the eyes, which would improve vision even more.” And thus, spectacles were born!