“…if you have a year or two to apply yourself to all that is necessary, I would hope that we might see, by your efforts, if there are animals on the moon…” (René Descartes, 1629)
With this alluring suggestion, René Descartes invited a young instrument maker to join him on an unprecedented and secret project that promised to revolutionize early modern astronomy. Descartes believed he had conceived a new kind of telescopic lens, ground to the shape of a hyperbola, that would surpass anything ever to come from the hands of the glass-working craftsmen of the era. This study traces the inception, development, and finally the collapse of this ambitious enterprise, and examines Descartes’ lens in the broader context of seventeenth-century optics, instrumentation, and mechanical craftsmanship. The history of the device sheds light on the history of telescopes in the period, on the relationship between instrument makers and mathematical adepts, on the mechanical philosophy and its connection to machine, and on the thought and work of Descartes himself.