To: atm{at}shore.net
From: Jim Burrows 
Reply-To: Jim Burrows 


At 06:10 2003-01-11 -0500, Scott Ewart wrote:

>WHY is it possible to see such small detail?

Maybe HOW is a better question...

Some facts about our optical instrument (the human eye) from W. J. Smith,
"Modern Optical Engineering", 1966.  On p. 104, the focal length
of the eye's lens is 17.1 mm, and "The cones of the fovea are 1 to 1.5
um in diameter and are about 2 to 2.5 um apart."  On p. 105,
"Normal visual acuity is considered to be ... one arc minute." 
1' = 290 urad, times 17.1 mm gives about 5 um, so the smallest object that
can be recognized has to hit several cones.

BUT, on p. 107, "The narrowest black line on a bright field that the
eye can detect subtends an angle from 1/2 to one second of arc.[!]" 
1" = 5 urad; the width of the black line on the retina is only .09 um,
and the brain must be able to decode the output of the cones to say there's
a black line there.  Two neat contraptions sitting there in our heads.

         -- Jim Burrows
         -- mailto://burrjaw{at}earthlink.net
         -- http://home.earthlink.net/~burrjaw
         -- Seattle N47.4723 W122.3662 (WGS84)

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