Girolamo Sirtori (ca 1589–ca 1660) and His Telescopes: Practice without Theory

Abstract In 1618 Girolamo Sirtori published his book, Telescopivm: Siue Ars Perficiendi Novvm Illvd Galilaei Visorivm Instrumentum ad Sydera. The Telescopium is a workshop manual, one of the most detailed optical handbooks published at that time. Sirtori constructed several telescopes but acknowledged that they performed poorly. He consulted authors in the field of optics as sources of optical knowledge but maintained a critical stance since he realized that none of the optical authors had contributed anything of substance to the body of optical knowledge. The Telescopium exposes the nature of the tension between theory and practice: how did it happen that authors who wrote on optics failed to construct advanced telescopes, while practitioners who successfully constructed telescopes good for terrestrial and naval usage, were unable to improve the performance of these devices? The construction of a high-quality telescope requires theories for determining the interplay among material and optical features and methods for controlling these relations. Such methods are based on optical theories which are categorically different from the “hands on” skills applied in the spectacle craft industry. We exhibit the properties of spectacle and telescope lenses which Sirtori used and discuss the optical setups of his telescopes. We ask: what is an optical theory? Did Sirtori have a theory to explain how telescopes function? We argue that Sirtori’s Telescopium is grounded in empirically derived rules of thumb from the spectacle-making craft and did not develop into a coherent instrumental theory capable of advancing telescope construction.