dioptra

from The Collaborative International Dictionary of English v.0.48

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]