This instrument, made by Augustine Ryther, is one of the oldest models of theodolite extant. It comprises a horizontal circle with a degree scale on the circumference and an inscribed shadow square. At the center pivots a diopter with a magnetic compass. The compass carries a holder for a vertical semicircle with a degree scale, shadow square, and viewer. The plane of the vertical semicircle rotates jointly with the diopter underneath, allowing the simultaneous determination of the azimuth angle and zenith angle of a given point, i.e., its spatial coordinates. The instrument, used for surveying, matches the one introduced in the second half of the sixteenth century by Leonard and Thomas Digges. Highly probable provenance: Robert Dudley bequest to the Medici collections.