205, [2] pp. Portrait frontispiece. 19x12 mm, original gilt-stamped vellum binding. First Miniature Edition.

First published in 1636, some twenty-one years after it was written, the letter, to his friend and patron Madama di Lorenza, argues for the harmony of religion and science. "For many years this tiny book which is only half the size of an ordinary postage stamp was considered the smallest printed from movable type" - Bondy pp. 95-96. The text is set in the exceedingly small "occhio di mosca" (fly’s eye) Dantino type created by Salmin for their miniature edition of Dante (1878). Welsh 2936.