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    Re: Perpendicularity
    From: George Huxtable
    Date: 2004 Oct 12, 19:13 +0100

    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
    contact George Huxtable by email at george@huxtable.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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