Henry Dreyfuss, Designing for People (1955)
George Miller, “The magical number seven, plus or minus two: Some limits on our capacity for processing information” (pdf) (1956)
The human capacity for remembering only seven bits of information, +- 2.
Paul Fitts, “The information capacity of the human motor system in controlling the amplitude of movement” (pdf) (1954)
The source of Fitts’ Law.