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"Aidan Karley" <doIlookDAFTenoughTOpost@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
wrote in message
news:VA.00000c6d.2b972e2c@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> In article <hiLJf.806978$xm3.134801@attbi_s21>, George wrote:
>> Perhaps they were confused by the double refraction of the calcite lens.
>>
> Nope. IIRC (but I may have to get Fortey's Blue Book off the shelf.
> Or Clarkson's tan book, for that matter.) the optical axis of the calcite
> crystals is parallel to the triclinic axis of the calcite. Uniaxial
> mineral, and isotropic in that direction. Hmmm, I had to do too much
> spinning crystals in my mind's eye for that. Better check.
> Wikipedia isn't very informative.
>
> --
> Aidan Karley FGS
> Aberdeen, Scotland,
> Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233
You are right, of course about the optical orientation. I said that
earlier a bit tongue-in-cheek.
Another critter that has calcite lenses in its 'eyes' is the brittle star
(yes, believe it or not, they do have eyes). Here is a link to research
being done on that interesting animal (I have a large one in my marine
aquarium):
http://www.aip.org/pt/vol-54/iss-10/p20.html
www.photonics.com/spectra/tech/XQ/ASP/techid.1234/QX/read.htm">http://www.photonics.com/spectra/tech/XQ/ASP/techid.1234/QX/read.htm
I was always amazed that my brittle star would hide in the live rock if I
came near the aquarium, and wondered if it was because it had receptors of
some sort that could distinquish variations in brightness (perhaps I was
shading the light in some way that it could detect). I now know that it
likely can actually see me. For such a "primitive" animal to have this
capability astonished me when I found this out.
George
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