'diagonal opera glass' definitions:

Definition of 'diagonal opera glass'

From: GCIDE

Polemoscope \Po*lem"o*scope\, n. [Gr. ? war + -scope: cf. F. pol['e]moscope.] An opera glass or field glass with an oblique mirror arranged for seeing objects that do not lie directly before the eye; -- called also diagonal opera glass, or side opera glass. [1913 Webster]