Note 422 — TREATISE OF NAVIGATION OF THE CHEVALIER ANTONY PIGAFETTA.
Henry Edward John Stanley Stanley translation
This means the arm of the instrument used; it might be the meteoroscope of Regiomontano, which had a cross in the middle: or an astrolabe like it; or the common astrolabe with a dioptron, or mediclino, as Pigafetta calls it, placed on the equator. Note, Milan edition.