Crossword Genius

Only about one-sixth of these in humans is visible (4)

Ross

I believe the answer is:

eyes

(Other definitions for eyes that I've seen before include "They have pupils" , "Ophthalmologist's speciality" , "Regards -- features of a potato" , "Looks at - organs" , "''Drink to me only with thine ...'' (Johnson)" .)

I've seen this clue in The New York Times.
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