polemoscope. An opera or field glass with an oblique mirror arranged for seeing objects not directly before the eye. “The detective used a polemoscope to observe the suspect unobtrusively

This device is usually a small monocular with a prism,
so that the user appears to be looking away from the
suspect, to avoid alerting suspect. They used to be
advertised in pulp magazines, called "See-backoscope".

Presumably from Greek polemos = war as in polemics. Perhaps they were once used on battlefields.

Bingley

Dear Bingley: I didn't get the point of your post until just now. In WWI, it would have been suicidal to put your head up above trench to use binoculars. So a 45degree mirror
overhead allowed safe use of binoculars. From Internet:

[page 185] Observation from behind cover or protected from enemy
sight has always been a necessity in the military. The oldest device
for this purpose is the Polemoscope, a stand with two parallel pairs of
mirrors at 45 degrees to the tube. 'Hevelius maintains that he had
invented this Polemoscope in 1637' (Pisko 1869, where on page 51 is a
picture of such an instrument. See bibliography.)