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    Re: Old Sextant on German money
    From: Alexandre Eremenko
    Date: 2007 Jan 28, 10:28 -0500

    
    On Sat, 27 Jan 2007, Robert Eno wrote:
    
    > I visited Germany about 10 years ago, and again a few years later. Of all
    > the souvenirs I took back with me, guess which one I still have safely
    > squirreled away?
    
    Sure. So did I. But I never seriously examined this
    picture before. The most curious part is
    the strange split index mirror which I've never seen
    before in the museums, e-bay or books.
    
    I know the Maskelyne proposal to make half of
    the index mirror silvered and half black.
    The silvered part for stars, the black part for the Sun.
    The purpose of the black part is to eliminate
    the second reflection (in a normal back silvered mirror,
    both surfaces reflect, and the front surface creates another
    much weaker image. Maskelyne's idea was to use for the Sun
    the front unsilvered surface only, because the light from
    the Sun is strong enough).
    But as I understand this Maskelyne proposal, the mirror
    should consist of one piece, only half of it
    silvered and another half blackened.
    
    In this particular sextant, we clearly see
    two separate mirrors.
    And they even do not look parallel!
    
    Also, the absence of horizon filters looks very strange.
    This is not a "cheap octant" for altitudes only;
    this is definitely a high-end sextant/pentant whose main
    purpose in XVIII century would be the lunars.
    
    Another feature I do not understand (though I saw it on
    many e-bay sextants) is a large flat disc surrounding
    the reading microscope. What is its purpose?
    Some older sextants frequently have it.
    
    Alex.
    
    
    
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