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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
Examples from the Web for 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