The telescope was crucial in bringing about a methodological shift in seventeenth-century natural philosophy. It legitimized the use of observational aids in the study of natural phenomena, and thus furthered the very role of observation in seventeenth-century inquiry. Nonetheless, many questions about its emergence, and its interaction with the evolving science of optics, remain unanswered. Isaac Beeckman's notes carry all the potential to shed light on a uniquely early and local reception of the telescope. In this chapter I will analyse how Beeckman gradually developed an understanding of the working principle of the telescope.

My analysis partly picks up on the recent historiography of the telescope. I will argue how Beeckman's familiarity with late-sixteenth-century optical innovations is larger than his modern editor, Cornelis de Waard, assumed. Still, if these innovations in optics were characterized by an increased attention for the mechanism of refraction of light in lenses, Beeckman's understanding of the telescope was only partially embedded in these. From his first notes onwards, Beeckman's reasoning based on refraction was complemented with a working principle resulting from a physical interpretation of the agents of vision, species, borrowed from other branches within the multifaceted sixteenth-century optical tradition. In Beeckman's early notes, these distinct contributions blended into a coherent whole. Interestingly, it was chiefly his physical interpretation of species that Beeckman subsequently merged in his maturing mechanistic ideas, arriving at a conception of the telescope that was original, and that was surprisingly adequate, too. It allowed Beeckman to understand the telescope as a device that brings remote things nearby, rather than an instrument that produces larger images.