Abstract

The Royal Society possesses three long-focus simple lenses of diameters 195, 210 and 230 mm, all inscribed with the signature ‘C. Huygens’ and various dates in the year 1686. These prove to have been made by Constantine Huygens, the elder brother of the famous Christiaan Huygens. All three lenses have been examined by a variety of physical and chemical methods, both to define their optical characteristics and to establish the composition of dated samples of late-seventeenth-century Continental glass. The focal lengths of 37·9, 50·1 and 65·2 metres found by combination agree with Huygens' own values of 122, 170 and 210 feet respectively, and are so great that practical employment of the lenses in aerial telescopes has rarely been achieved. All are made from the same very poor glass—a heterogeneous and discoloured potash-rich ‘forest glass’—with a refractive index of 1·516, a costringence of 60, and a density of 2·5 g cm−3. The three lenses were ground with just two concave laps, of radii of curvature 27·11 and 71·15 metres, one lens being plano-convex to a high degree of accuracy. A claim by previous investigators that one lens was a ‘light flint’ glass has been disproved