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
Historical Examples of naked eye
Compare the naked-eye appearances of the cultures from day to day.The Elements of Bacteriological Technique
John William Henry Eyre
Of those that are known, perhaps one in twelve develops into a naked-eye comet, and in some years six or seven will be discovered.
This nebula is a naked-eye object near Eta Andromed, and it is often mistaken for a comet.
Stars visible without the aid of a telescope are referred to usually as "naked-eye stars."Astronomy for Young Folks
Isabel Martin Lewis
These were before the invention of the telescope, and consequently contained only naked-eye stars.The Astronomy of the Bible
E. Walter Maunder