The resolving power of the human eye is 0.0003 of a radian or an arc of one minute (1/60th of a degree), which corresponds to 100 microns at 10 in. (confused yet?) A micron is a thousandth of a millimeter, hence 100 microns is 0.00397, or less than four thousandths of an inch, the human eye can detect a bright light shining through an aperture of only three or four microns
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