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The Invisible Man

From TARDIS Index File, the free Doctor Who reference.

The Invisible Man by H.G. Wells seems inspired by Wells' experiences on the planet Karfel with the Doctor. Briefly, the Doctor used kontron crystals to alter time slightly, so that his image would lag behind his physical self by a few seconds. This gave the illusion of invisibility. (DW: Timelash) The Master met Griffin, the villainous invisible man himself, in the Land of Fiction. (DWM: Character Assassin)

Earlier in his personal timeline, however, the Doctor would claim to have already done experiments with Wells so that he (Wells) could make himself invisible. (BBCR/MA: The Ghosts of N-Space)

A number of actual invisible species, such as Visians and the inhabits of Spiridon do exist.
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