Sight unassisted by an instrument such as a microscope or telescope. For example, These insects are too small to be seen with the naked eye. This expression was first recorded in 1664.
Examples from the Web for naked eye
Within the area of the Square Argelander counted thirty naked-eye stars.A Field Book of the Stars|William Tyler Olcott
These were before the invention of the telescope, and consequently contained only naked-eye stars.The Astronomy of the Bible|E. Walter Maunder
Microscopic examination confirmed my interpretation of the naked-eye appearances.Parasites|T. Spencer Cobbold
Compare the naked-eye appearances of the cultures from day to day.The Elements of Bacteriological Technique|John William Henry Eyre
Immediately below Equuleus are the stars α1 and α^272 Capricorni, seen as a naked-eye double to the right of and above β.Half-hours with the Telescope|Richard A. Proctor