Testing telescope optics of seventeenth-century Italy

In the course of the last two decades, a significant part of the 17th Century telescope optics remaining in Italy (mainly at the Institute and Museum of the History of Science in Florence) has been submitted to optical tests in research laboratories, in order to assess the lens quality and retrieve any information that might be useful to science historians. The investigation is still in progress. A number of interesting results have already come to light, showing that the lens quality that could be obtained at that time was extraordinarily good; in particular, the "broken objective lens" of Galileo housed in Florence turned out to be almost perfect as to optical performance. Such a high quality was achieved both in terms of lens surface, figuring, and polishing, and in terms of purity and homogeneity of the glass material. In this paper, the most significant outcome of this research is reviewed, and results of recent measurements are reported. In particular, aspects related to the glass quality are discussed, pointing out the critical problem of glass homogeneity. Defective glass could not be detected until the lens was polished and tested, and even then the causes of failure could probably not be understood. Current analysis is devoted to find out the chemical composition and the physical properties of the glass material, as an aid to locating the lenses and the telescopes more precisely within their historical framework.