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on 5/14/04 5:58 AM, Jim Thompson at jim2@JIMTHOMPSON.NET wrote:

> Kieran,
>
> Excellent case report; thank you.  I made the stupid mistake (once) of
> starting to view the sun to check index error with the index arm set to zero
> degrees after viewing the horizon, without swinging down the horizon mirror
> shades.  Fortunately I sensed the very bright light just starting to flare
> in the periphery of my field of vision before I got a retinal blast, and
> quickly turned away.  I did not have any immediate or subsequent symptoms.
> I have had laser surgery a couple of times: it really flared my vision green
> for several minutes after the procedure, and was far more stimulating than
> that brief peripheral solar exposure.
>
> Being a physician you stimulated my professional curiosity, so I did a
> brief, limited literature search on Pubmed to learn more about solar eye
> injury.  I posted the literature search results in a separate email.  I
> found not a single reference to navigational sextants in Pubmed, for any
> health reason whatsoever.  Interestingly the world's medical literature
> appears to be completely silent on navigatonal sextants!  No news is good
> news, I guess.
>
> I get the sense that humans are well adapted to moderate sun exposure.
> Although sun-gazing can definitely cause damage, much of it appears to be
> reversible, probably because sufferers are quick enough to turn away soon
> enough.  I am by no means expert on this issue, but I get the sense from my
> reading so far that for significant damage to occur, the eye has to be
> exposed to direct sunlight for relatively lengthy periods, or has to suffer
> repeated exposures over long periods of time.  My conclusion is that human
> eyes are well adapted to deal with limited exposures to direct sunlight, but
> that we do not want to push the edge of that envelope.
>
> As Kieran pointed out there are risk groups, such as the intoxicated.  But
> hunters, sailors and other people who occupationally stare at bright skies
> are also at risk of acute and chronic eye damage.
>
> Sunlight can acutely damage all the tissues of the eye in different ways:
> - Lids and conjunctiva: sunburn (UV).
> - Cornea: sunburn (UV damage, same as welders' flash).
> - Lens (UV, heat).
> - Vitreous.
> - Retina (heat, UV, and components of light).
> There is also a link between UV exposure and ocular melanoma.
>
> I think it is clear that, as with all things in health, minimizing exposure
> to noxious stimulus is ideal, so scrupulous eye care is wise when using a
> sextant, or when out in bright sunlight for any other reason.  I recommend
> sunglasses between sights, as Kieran suggested, lots of sunscreen, minimal
> observation time, careful attention to the condition of the shades, deep
> caution when swinging up to view the sun, and very careful attention to use
> of the shades.
>
> Jim Thompson
> jim2@jimthompson.net
> www.jimthompson.net
> Outgoing mail scanned by Norton Antivirus
> -----------------------------------------
>
Thanks to Jim, Kieran and George for picking up on this side of the thread.
Armed with your posted information, those involved with sextant
manufacturing can breathe a little easier in our litigious society.
Ken Gebhart

   

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