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THE CRAFT OF THE LENS
In 1609 when Galileo Galilei trained a pair of lenses on the night sky, the telescope was an innovation but the lens was not.1 Since the thirteenth century, artisan-opticians had made spectacles for the monks and elites in Italy, France, and Germany.2 The artisanal craft of lens making, which grew to serve the needs of spectacle makers, could not, however, provide enough lenses of requisite quality to meet the explosive demand created by the new viewing tubes.3 So powerful was the attraction of the new instruments that by 1617 Galileo's intimate friend Giovanfrancesco Sagredo was disappointed to discover that his correspondents in distant India were not surprised by the telescope he sent them-they already had enough to go around; by 1624 one wealthy savant in Europe had purchased more than forty of them.4 The instant demand for the new optical devices-for both terrestrial and celestial use-necessitated lenses, many lenses, ground to ever more demanding specifications, lenses from the size of a pinhead to lenses that required commandeered naval vessels for transportation.
What were the craft practices for lens making that antedated the development of the telescope? The earliest detailed discussions of the process appear in William Bourne's tract on enlarging lenses (composed around 1580) and Giambattista Delia Porta's Magia Naturalis printed (in its most commonly cited form) in 1589, a book that went through more than twenty editions and was widely translated.5 In book seventeen, entitled "Of Strange Glasses," Delia Porta explains:
It remains now to teach you how Spectacles and looking glasses are made, that every man may provide them for his use. In Germany there are made Glas-balls, whose diameter is a foot long or there abouts. The ball is marked with emril-stone round, and is so cut into many small circles, and they are brought to Venice. Here with a handle of wood are they glewed on, by Colophonia6 melted: And if you will make Convex spectacles, you must have a hollow iron dish, that is a portion of a great sphere, as you will have your Spectacles more or less Convex; and the dish must be perfectly polished. But if we seek Concave Spectacles; let there be an iron ball, like to those we shoot...