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    Re: sextants and micrometers
    From: Steven Wepster
    Date: 2003 Aug 26, 17:27 +0200

    George,
    
    I was tought the use of the device commonly known under the names 'nonius'
    and 'vernier' first under the former name. Whether is is called nonius or
    vernier might be time and place dependent.
    
    Concerning the micrometer, my first reaction was like yours: they probably
    meant the reading-lens. But I'm afraid that our thinking is governed too
    much by ignorance. Hopefully someone else is able to reflect on the
    correctness of your interpretation.
    
    The accuracy of the scale (and the reqading of it) as described is
    consistent with the scale and reading of my old sextant of ca.1945 which
    still has a vernier. The divisions on the vernier correspond to a reading
    of 0.2' or 12", and you can guess to half of that or 6". Any finer
    subdivision is not practicable. For reading the vernier, adequate
    illumination and the use of the reading glass is of much help.
    
    But on the other hand, various forms of micrometers have been fitted to
    telescopes by astronomers since Gascoigne (1640) and Auzout (1666), to
    measure distances between various objects in the field of view of the
    telescope. I have never heard of a micrometer fitted to the telescope of a
    _sextant_, and I do not see any practical use for it. It would introduce
    additional reading error (reading of the scale and of the micrometer) and
    it would also introduce a large probability of blunder (add or subtract the
    two readings?) while nothing is gained.
    
    
    George, may I take the opportunity to congratulate you with you being
    distinguished with a Fellowship of the Royal Institute of Navigation, for
    your "status as an acknowledged authority on historical aspects of
    navigation, in particular [your] critical contribution towards the
    Institute's involvement in the BBC TV programme 'The Ship'". The happy
    event happened almost a year ago. Well deserved, George, and I hope that
    you will share your sharp thoughts on maritime-historical matters with us
    for many years to come.
    
    Steven.
    
    
    

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