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    Re: Sextant mirrors
    From: Fred Hebard
    Date: 2007 Oct 9, 14:19 -0400

    On Oct 9, 2007, at 9:58 AM, George Huxtable wrote:
    > 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
    > 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.
    Hmm, I wonder if this shows up in artificial horizons, which is where
    I recall (vaguely) having seen it?  No time to check now, but I'll see.
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