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    Re: Perpendicularity
    From: George Huxtable
    Date: 2004 Oct 12, 19:45 +0100

    A few mimute ago, I wrote-
    "Two correspondents (Doug Royer, Gary Harkins), have informed us that the
    simple test for perpendicularity of the index mirror of a sextant (by
    checking that the arc aligns with its reflection) will work only if the
    mirror is back-silvered.
    They have given no references or reasons for the failure of this method
    with a front-silvered mirror.
    I have been racking my brains to imagine what could cause either of the
    methods that were described to be any better or worse depending on which
    surface the mirror was silvered. Just because I can't think of such a
    reason, that doesn't inply that there isn't one.
    So I ask: is that claim actually true? Has anyone tried it out with a
    front-silvered mirror? Did it fail, and what were the symptoms? Has this
    failure been explained, or even mentioned, in a publication? Can any list
    member explain the cause, please, for our benefit?
    I'm not (at present) challenging the truth or otherwise of that claim, but
    I would like to be convinced, if only to distinguish between fact and
    And now, thanks to a communication just received from Doug, " Tamaya MS-733
    manual", I am starting to see light!
    He quotes from the Tamaya manual, about front silvered index mirrors-
    If you need to replace your Index mirror with a mirror that is silvered on
    the front surface,the silvering will be located forward of the axis of
    rotation of the sextant arm.With this configuration,the normal method of
    useing the reflected arc will not be adequate.You will notice that as the
    eye is moved above the sextant,the appearance of the reflected arc
    changes.The only correct observation point is with the eyes on a level with
    the sextant frame itself.however,in this position,nothing of the reflected
    arc can be observed..."
    Note that he is NOT referring to ALL front-silvered mirror sextants, ONLY
    those in which an index mirror (presumably rear-silvered) has been replaced
    by a front-silvered mirror. In that case, if the glass occupied the same
    position as before, the effective reflecting surface would be shifted
    forward, by something less than the thickness of the glass. And no longer
    would the rotation axis of the sextant lie exactly in that reflecting
    plane. I can see how that might change the appearance of the arc as
    reflected in the index mirror, so as to spoil the exact alignment.
    Presumably a responsible sextant-maker, introducing a design with
    front-silvered mirrors from new, would take steps to ensure that the axis
    passed through the reflecting surface, in which case either method of
    aligning the index mirror would suffice. Would our sextant experts care to
    Doug's original mailing said-
    "It is stated that all front coated index mirrors can not be checked for
    perpendicularity in the old way of useing the arc's relected image."
    In the light of this more recent information, perhaps than now needs some
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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