11 June 2006

The eyes have it!

I’ve been wandering around finding out about a neat little arthropod called a Trilobite — particularly their eyes. A few snippets...

Not only is the trilobite eye made of pure calcite (transparent Ca carbonate) which has a precisely aligned optical axis to eliminate any double image that would have formed, it is also a "doublet" of two lenses affixed together in order to eliminate spherical aberrations (as found in ground-glass lenses)! Trilobite eyes are massively arrayed in semicircular banks & even almost-circular banks of up to 30-60 lenses per row, each with its own individual retina
Ordovician trilobites such as Pricyclopyge binodosa and Jujuyaspis keideli are said to have had a “large visual field” with “close to 360-degree vision” and “could see anteriorly [backwards], laterally [sideways], dorsally [upwards], and even downwards and backwards,” from one position. Further, it has been shown that another Ordovician trilobite, Dalmanitina socialis, actually has a doublet lens arrangement where the top calcite lens has a “conspicuous central bulge, the cause of bifocality, which is a unique optical feature in the animal kingdom.”

Pretty recent for us, too. And so on. These little bony-looking greeblies are responsible for...

[...] the development of glasslike lenses that correct for spherical (and chromatic?) aberration, the density of seawater & the function of bifocality [...]

...and according to one bloke...

Trilobites had solved a very elegant physical problem & apparently knew about Fermat’s principle, Abbe’s sine law, Snell’s laws of refraction & the optics of birefringent crystals [...]

...that’s quite impressive for a Lower Cambrian to Permian arthropod (exoskeletal with symmetrical, segmented bodies; think cockroaches & other insects, spiders, ants, crabs, lobsters, centipedes and other chilopods, etc)...

However, amongst all of this little critter’s many wonders, the eyes particularly stand out. So much so that many classifiers simply raise their mental hands and avoid the problem.

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