The Collaborative International Dictionary of English v.0.48:

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]