Illustrated Companion to the Latin Dictionary/Dioptra

This article incorporates text from a publication now in the public domain: Rich, Anthony (1849). The illustrated companion to the Latin dictionary, and Greek lexicon. p. vi. OCLC 894670115. https://archive.org/details/illustratedcompa00rich. 

DIOP'TRA (δίοπτρα). A geometrical instrument employed in measuring the altitude of distant objects; for taking the levels of a source of water intended to be conveyed to a distance by means of an aqueduct, and similar purposes. Vitruv. viii. 5. 1.