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    Re: Sextant mirrors
    From: George Huxtable
    Date: 2007 Oct 9, 14:58 +0100

    I had written-
    
     No matter
    > which side the glass is coated, the thickness of the glass does not give
    > rise to multiple images, not at all. The refractive index and the
    > thickness
    > of the glass make no difference whatsoever. With rear silvering, some
    > light
    > will have been reflected from the front surface, some from the back, but
    > to
    > the observer's eye the two images will EXACTLY coincide.
    
    and Nicolas de Hilster added-
    
    I played around with my David White & Co 1941 US Navy quintant. I
    removed the telescope and observed a high contrast image through the
    index-mirror only. In this way I saw two ghost images, one on either
    side of the main image (something Van Breen already mentioned in 1662
    when he described his spiegelboog [Dutch for mirror-staff] in his book
    Stiermans Gemack). Now when I put the telescope back in place and
    observe the same image through the horizon mirror and index mirror, so
    using double reflection, the two ghost images seem to have disappeared
    or at least have become too faint to distinguish. So it is the
    combination of two mirrors that makes the multiple images disappear for
    the eye (so they do not exactly coincide as George said). When using the
    spiegelboog one has to deal with those annoying multiple images (and so
    had Robert Hooke with his single reflecting instrument in 1666).
    
    ================
    
    I think that Nicolas has got the explanation wrong, however. The single
    image you see with a back-silvered mirror occurs even with a single
    reflection; it doesn't need double reflection. But it does need PARALLEL
    LI|GHT, such as you get from an object in the sky. Unless what you are
    looking at is a long way away, you will see two images, a bright one and a
    faint one, just as you get with your own reflection when looking in a
    domestic mirror.
    
    So I ask Nicolas how far away was the source of the "high-contrast
    image"contact George Huxtable at george---.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. that he refers
    to?
    
    As for the Spiegelboog, I have read with interest Nicolas' own account of
    the instrument in Bulletin of the Scientific Instrument Society (though I no
    longer have that issue to hand), heard his lecture about it, and seen him
    demonstrate the replica he has made. As I recall, it involves aligning with
    the horizon the reflected image of a wooden sight-vane, which is just
    obscuring the Sun. And though the Sun is at infinity, the vane isn't, and
    that's why multiple images of its edge occur, when seen through the mirror.
    Have I remembered it right?
    
    George.
    
    contact George Huxtable at george---.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    
    
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