Academic journal article Transactions of the American Philosophical Society

The Development of Spectacles in the Later Middle Ages and in the Renaissance

Academic journal article Transactions of the American Philosophical Society

The Development of Spectacles in the Later Middle Ages and in the Renaissance

Article excerpt

The first negative spectacles

As we noted above, breaking glass balls produced many meniscus-shaped glass plates. To make positive lenses for presbyopic eyes, one needs to grind only the concave side plane. But it was also easy to see that by grinding the convex side plane, one would get negative lenses for myopic eyes. Therefore, the making of negative glasses was no more complicated than making positive ones.

Nevertheless, the earliest written documents relating to negative spectacle glasses date only from the middle of the fifteenth century21 One of the documents is a letter dated 21 October 1462, from the duke Francesco Sforza of Milan, to his ambassador in Florence, Nicodemo Tranchedini of Pontremoli. The letter states that he had been repeatedly asked to procure spectacles from Florence because they were of much better quality than spectacles made elsewhere in Italy. He ordered three dozen glasses packed into small cases. One dozen was for younger people, to help them view distant objects (negative glasses), one dozen was for older people, to help them see short distances (positive glasses), and one dozen was for normal eyes (plane glasses).

Four years later, three months after the death of Duke Francesco, there was a request from his son in a letter, dated 13 June 1466 to the same ambassador in Florence, to send him more spectacles according to the following list:

* 15 pair of eyeglasses for 30, 35, 40, 45, and $0 years, of the best quality

* 15 pair of eyeglass for 40, 45, 50, 55, 60, 65 and 70 years

* 10 pair for medium sight for young people (negative)

* 10 pair for distance viewing for young people (negative)

In both of these letters it is obvious that negative spectacles for myopic eyes were regarded as entirely ordinary objects, and ones that spectacle makers would be expected to have in stock.

Although there is no extant documentation concerning negative lenses from an earlier period, this silence is not a sign that they did not exist in the fourteenth century. Because we know now how easy they were to make, the assumption would be that they, too, were manufactured in the fourteenth century. Negative glasses for myopic eyes, even those dating from the sixteenth century, are very rarely found today in collections, but this apparent shortage can be explained by the fact that the demand for them was much less than that for positive glasses for presbyopic eyes. This was probably because nearsightedness is not as common as farsightedness, for the latter condition is the fate of everyone attaining 45-50 years of age. We can therefore assume that the proportion would be around 100 positive glasses to each negative glass, with the possibility none of them even survived.

Spectacle-glass production in northern Italy

Venice's preservation of its monopoly on spectacle glassmaking could not have been sustained for a very long time. As we already know, in 1313 a chronicle mentioned that the monk Alessandro Spina could make spectacles, and was happy to teach the art to anyone who was interested. There were only two difficulties. First was the necessity of blowing a more or less spherical glass ball. Its thickness was critical; if it was too thick, the ball quickly elongated to a pear shape due to its weight. But if it was too thin, when the plane was ground, the resulting plates had much too small a diameter. The second problem lay with the cristallum. It was especially important that the material of which the oven was composed came from secondary clay deposits, and that the glassblowers had the necessary natron, and not the potash, which was used north of the Alps, for the melt.

To what extent northern Italian towns had access to Alexandria and to the salt from Wadi en Natrun is not known, but it seems likely that the superior maritime power of Venice was dedicated to preserving this monopoly. The three spectacles from Wienhausen suggest that the Venetians had been successful in this endeavour for at least several decades. …

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