3 eyes do not an imaging system make

[P3D Peter Abrahams <telscope@xxxxxxxxxx>]

>>Any zoologists out there aware of animals that have more than two eyes? Where
>>do they keep the "extras"? And what's their visualizing advantage thought to
>>be? 
>Plenty. Spiders have multiple *kinds* of eyes (i.e. some simple, some
>compound). The different kinds of eyes are used for different purposes.
>Scallops have numerous eyes, all around the edge of the shell, which they
>may use to sense danger so they can swim away. (Thus illustrating that
>bivalves are not as limited as one might think.)
Further illustrating that eyes can be more like light detectors than imaging
systems, and that an eye with a miniscule brain behind it might not see 3d
like we expect.
(Not that either author quoted thought otherwise.)

Slightly off-topic here are any of the common scallop's 60 eyes: they are
catadioptric, 
with a lens that has a reflecting membrane on the back surface.  The cornea
acts to converge light at its center and diverge light at the edges.  The
combination focuses light on some type of fovea.  Excuse my lack of citation
here, I can dig it up.

Gigantocypris, a plankton, has no lens, but a concave reflecting membrane
that focuses onto something like a fovea mid-eye, in each of two eyes.

Neither of these critters can form an image that we would deem acceptable.  
So, why not nominate them to be art critics, or judges at a stereo photo
contest?
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telscope@xxxxxxxxxx (Peter Abrahams)          
the history of the telescope, 
     the prism binocular, and the microscope


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