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    Re: Telescope danger to sight.
    From: Ken Gebhart
    Date: 2004 May 14, 10:54 -0500

    on 5/14/04 5:58 AM, Jim Thompson at jim2{at}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{at}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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