Monocle

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

The First Doctor sometimes used a monocle. He used it to examine the Sensorites. (TV: The Sensorites) He used it when he confronted Fomal on Wengrol. (PROSE: The Sons of the Crab)

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)