Monocle

A monocle was a type of glass lens used for only one eye.

The First Doctor used a monocle when he encountered the Sensorites, (TV: The Sensorites) Fomal, (PROSE: The Sons of the Crab), the Korads (PROSE: Peril in Mechanistria), the Voord (PROSE: The Fishmen of Kandalinga) and the Testimony Foundation. (TV: Twice Upon a Time)

The War Lord von Weich used a monocle to implant suggestions into the minds of his human subordinates. (TV: The War Games)

The computer Gus used the image of a monocle as its avatar. (TV: Mummy on the Orient Express)