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Phrase(s): *the naked eye
the human eye, unassisted by optics, such as a telescope, microscope, or spectacles. (*Typically: appear to ~; look to ~; see with ~; visible to ~.) • I can’t see the bird’s markings with the naked eye. • The scientist could see nothing in the liquid with the naked eye, but with the aid of a microscope, she identified the bacteria. • That’s how it appears to the naked eye.
McGrawhill's American Idioms And Phrasal Verbs