What is "dioptra"

Diopter \Di*op"ter\, ||Dioptra \Di*op"tra\, n. [L. dioptra, fr. Gr. ?. See 2d Dioptric.] An optical instrument, invented by Hipparchus, for taking altitudes, leveling, etc. [1913 Webster] ||

n. (alternative form of diopter English)

A dioptra (sometimes also named diopter or diopter from ) is a classical astronomical and surveying instrument, dating from the 3rd century BCE. The dioptra was a sighting tube or, alternatively, a rod with a sight at both ends, attached to a stand. If fitted with protractors, it could be used to measure angles.