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    Re: Bris Sextant
    From: Alexandre Eremenko
    Date: 2005 Oct 31, 12:14 -0500

    Dear John,
    I will certainly measure the sizes of the glass pieces
    in my Bris sext., but I don't see any reliable way
    to measure the angles between them. I supose these angles
    can be COMPUTED instead, mathematically
    from my calibration results.
    I will also ask Bill, if he could make good
    pictures of my Bris sext. to post them on my web site.
    
    One problem with a Bris sextant is that it cannot be
    used for anything, and cannot be calibrated unless you have a
    natural horizon. And I don't have it:-(
    
    By the way, I remember about a year ago one member of this
    list mentioned successive use of a Bris sextant in a desert.
    I wonder what desert was this? You need an extremally flat
    desert...
    
    Alex.
    
    
    On Sun, 30 Oct 2005, John Rae wrote:
    
    > Thanks Alex for your information.
    > Yes, I made the test sextant from three pieces of plain glass, and used my 
    mountain sunglasses (<7% transmission), but even so, I was careful to block 
    the true sun with my hat brim and finger.  This may be OK for a test, but 
    obviously not for a working sextant.
    > I intend to use a colored glass for one of the elements of the next sextant 
    I make.  A local camera store that has an extensive stock of used cameras, 
    lenses and accessories sells used filters for about $10 each.  I hope to find 
    a suitable filter , and cut a rectangle from the circular part.  The most 
    common diameter seems to be 49 mm, but 52 and 55 mm are also available.  You 
    might try your local camera store.  I will also try a piece of polaroid with 
    the objective of reducing glare at the sea horizon.
    >
    > I was impressed with your calibration sights using your Bris Sextant, and I 
    expect that those might be approximately similar for other instruments, but 
    each would have to be calibrated individually.  The rainy season is upon us, 
    so I have to wait for a good time, maybe even until next spring before I can 
    get a good day.  (And I hope to be tied up to the dock!)
    >
    > Could I ask if you could measure the dimensions of your Bris Sextant, 
    particularly the gap at the top end between the two pairs of glass and the 
    length to the bottom apex, so that I could calculate the angle between the 
    glass elements?  Your report of 3 bright, 4 dim and 1 very dim suggests from 
    my tests that the angles between the two outer glasses are not the same, one 
    perhaps twice that of the second angle.
    
    
    

       
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