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    Re: Old Sextant on German money
    From: George Huxtable
    Date: 2007 Mar 10, 17:32 -0000

    Frank's brilliant bit of sleuthing to find those pictures of the Gauss
    instrument, and Wolfgang's description of it, are enabling us to put
    two and two together. It seems that Alex's description of what he
    could make out from the banknote were correct, and my own comments on
    the digital image were mostly wrong.
    So how did the thing work, then? I will offer a guess, for what it's
    Wolfgang wrote-
    "As far as I understand the description the Vize-Heliotrop is a
    equipped with an additional mirror which reflects the image of the sun
    to a
    distant point. It is not used for measuring then but for marking the
    position of the observer for another observer at the distant point."
    It seems to me, then, that it's intended to solve the same problem
    that faces distressed mariners, who are advised to use a mirror to
    flash reflected sunlight at a possible rescuer, in an attempt to
    attract attention. But how does he decide to angle the mirror, to
    ensure that the light reaches its target? Gauss's instrument solves
    that same problem, scientifically.
    To avoid confusion, let's call the man holding the instrument the
    observer, and the man at the distant point (another observer) the
    target, at whom the reflected sunlight is going to be aimed.
    To do its job, the normal to the mirror (that is, the line drawn
    perpendicular to its plane) must be in the plane containing the
    observer, the Sun, and the target, and it must bisect the angle
    between Sun and target, as seen from the observer. For that purpose,
    the mirror is added to a sextant, as a piggy-back on the index mirror,
    fixed to it, but twisted a bit in angle, as Frank's photographs
    clearly show. As I see it, it has to be carefully mounted to be
    perpendicular to the frame of the sextant, just like the index mirror
    is. But that added mirror plays no part in the the observer's
    proceedings; he simply aligns the sextant to look at the target point
    via the horizon mirror, and the Sun via the (shaded) index mirror, as
    though he was measuring the angle between them. There's no call to
    record that angle, however.
    Now the (filtered) light from the Sun passes, via the normal index
    mirror and the horizon mirror, into the telescope. Similarly, unshaded
    sunlight strikes the added mirror, with full brilliance, and is
    reflected along a different path; still in the plane of the sextant
    frame, but angled differently, depending on the offset angle between
    the index mirror and the added mirror. What value should be chosen for
    that offset angle ? As I see it, if it's made exactly equal to the
    (fixed) tilt angle of the normal to the horizon mirror, with respect
    to the telescope axis, then the light will be reflected from the added
    mirror, not toward the horizon mirror, but directly toward the target.
    In that way, it will fulfil the requirement of sending a flash of
    sunlight to the second observer at the distant target, and show him
    the position of the first observer, with his Gauss instrument.
    It may be worthwhile making the added mirror less than perfectly flat;
    perhaps very slightly curved or rippled, so as to diffuse the
    reflection a bit and relax the need for precise alignment.
    Does all that make any sense? Have I got it right? It does nothing to
    explain the curious mechanism that's shown by the horizon mirror,
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    To post to this group, send email to NavList@fer3.com
    To unsubscribe, send email to NavList-unsubscribe@fer3.com

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