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    Re: Sextant mirrors
    From: Nicol�s de Hilster
    Date: 2007 Oct 09, 17:00 +0200

    > George wrote:
    >  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 then I 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).
    > ================
    > The George replied:
    > 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?
    And now I feel pretty stupid to fall into this pitfall as we say in
    Holland. Forgot all about the parallax within the mirror! The image was
    indeed too close (just a few metres) to me. Still the phenomena does not
    occur with double reflection, so there must be a good reason for that...
    Van Breen did write: "... in de glase spiegels [...] somtijdts dry
    Sonnen werden ghesien, als de Sonne heel klaer schijnt, maer wat flaeu
    door de wolcken ghesien werdende, siet men meest maer twee Sonnen",
    which translates to "... in the glass mirrors [...] sometimes three suns
    are seen, when the sun shines very bright, but when seen weakly through
    the clouds, one often only sees two suns".
    At this very moment the sun is shining through a bit of clouds (it is
    one of those dull days in Holland), and there is only one image of the
    sun in my mirror (even the plain one that I used for the
    reconstruction). So I assume the quality of Van Breen's mirror's was
    either not very good, or he was a confused just as I was (maybe observed
    three images when testing his instrument with a candle).
    > 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?
    Apart from the name of the vane that is correct. One has to look through
    the sight vane at the sun and try to superimpose it on the horizon on
    top of the shadow vane, of which we will indeed see two images. It does
    however not affect the observation as one can easily see which edge is
    the proper one (it is the lower of the two) and the sun is that bright
    that it the contrast between the proper edge (fully dark) and the 'ghost
    edge' (faint) is getting even better (so the ghost edge is getting
    almost invisible).
    The article can be downloaded as pdf from my web site www.dehilster.info
    at the spiegelboog-page (with permission from SIS-bulletin).
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