Somnor Disc

Somnor discs (PROSE: Doctor Who and the Keys of Marinus) were small metal discs used by the Morpho of Morphoton on Marinus to make humanoid minds receptive to their hypnotic control. Combined with a sound, the discs allowed affected minds to see whatever the Morpho wanted.

The discs were placed on the First Doctor, Susan, Ian and Barbara as they slept during their first night. However, Barbara's disc slipped off and thus her mind was the only one unaffected when she awoke the next morning, able to see the buildings as rundown and the clothes as rags. She later smashed the Morpho and freed the city from their control. (TV: "The Velvet Web")