naked eye

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The eye unassisted by an optical instrument.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Eyesight, unaided by equipment such as a telescope or microscope.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the eye unaided by any optical instrument that alters the power of vision or alters the apparent size or distance of objects