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    Hermetically sealed sextant mirrors
    From: Paul Hirose
    Date: 2019 Aug 29, 14:43 -0700

    Yesterday at a library I came across "The Sextant Simplified" by Oswald
    M. Watts ("Master Mariner, F.R.A.S, F.R.I.N., Assoc. R.I.N.A., editor of
    Reed's Nautical Almanac"), 1976.
    Watt uses a curious term I haven't seen before. "The Hezzanith
    rectangular horizon glass is provided hermetically sealed (and therefore
    impervious to salt water or spray) in the more expensive types, and can
    be fitted as an extra in the cheaper models."
    And, "A Hezzanith rectangular index glass may be made hermetically
    sealed and impervious to the action of salt spray, and therefore the
    mirror never needs re-silvering."
    And, "Figure 7 shows the Husun circular index mirror, which hermetically
    seals the mirror from the entry of salt spray, and thus entirely
    prevents the deterioration of the mirrors from this cause."
    So how do you hermetically seal a mirror? I got a clue from — of all the
    unlikely places — a product description at restroomdirect.com: "Standard
    glazing is No 1 quality, 1/4" (6.4mm) thick plate/float glass, silver
    coated and hermetically sealed with uniform copper plating, and
    warranted against silver spoilage for 15 years. Mirrors meet Federal
    Specification DD-M-411C, ASTM C1036-91."

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