Academic journal article Transactions of the American Philosophical Society

The Reading Stones (Lapides Ad Legendum)

Academic journal article Transactions of the American Philosophical Society

The Reading Stones (Lapides Ad Legendum)

Article excerpt

AT SEVERAL MONASTERIES and churches in Germany and Switzerland, and in a few in France, we can still find liturgical art treasures dating back to the eighth century. France has fewer such specimens because these smaller art treasures were often destroyed during the French Revolution. Most of the sites are richly decorated with round stones such as white and brown rock crystal, amethyst, and almandine (or almandite).

A careful examination reveals that all of these stones have planoconvex surfaces, where the convex side is ground more or less distinctly aspherical (figures 5 and 6). Often the curvature of the convex surface is very strong. If we put such a stone with its plane surface on a script, then we see the writing more or less magnified (figures 7 and 8). Surely the magnifying power of crystals ground in this manner was recognized very quickly by their makers. We can therefore assume that this feature was already well known in Antiquity. The skilled monks in the High Middle Ages likewise knew this optical effect well, and during earlier centuries it was the only remedy against the weakness of old eyes, or what today we call presbyopic eyes.

But it was an imperfect solution to the ophthalmological problem. First, because such a stone needed to be placed directly on the text, it could only help for reading. For writing it was entirely useless. Secondly, it only magnified the letters, but they remained unclear on the retina. From an optical viewpoint, the reading stone was a magnifying glass and its refracting power was in the range of 30 to 40 diopters. …

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