Green’s Dictionary of Slang

eyes n.2

[the mirror extends the range of one’s eyesight]

(US prison) mirrors held through the bars of one’s cell and used to survey the outer world.

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[US]Bentley & Corbett Prison Sl. 6: Eyes Hand mirrors used by inmates to look down their row of cells [...] Inmates hold these mirrors out through the bars of their cells and position them so they can see in any direction.