nPolemoscopeAn 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.
Century Dictionary and Cyclopedia
npolemoscopeA perspective glass fitted with a mirror set at an angle, designed for viewing objects that do not lie directly before the eye: so named from its possible use in warfare to observe the motions of the enemy from behind defenses. Opera-glasses also are sometimes constructed in this way, to admit of seeing persons obliquely without apparently directing the glass at them.
Chambers's Twentieth Century Dictionary
nPolemoscopea perspective glass so constructed as to give views of objects not lying directly before the eye