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    Re: Prism levels to correct sextant tilt.
    From: JC Sutherland
    Date: 1999 Nov 01, 17:42 EST

    Geof,
    Some years ago I had a 'Hezzanith ' sextant which had a prism as an optional
    extra. I never had such a prism so I have no experience as to how it worked.
    However I have in front of me as I  write  a copy of the instruction booklet
    which came with it (priced 1shilling & 6 pence)!.
    Two prisms as attachments are described .
    First is a 'Wollaston prism' which divided the light from a star into two
    separated beams 16 mins apart and in observations the horizon is brought midway
    between the two images. This is said to be advantagous when the horizon is not
    well defined. The prism was attached between the horizon glass and the index
    mirror so  would not correct sextant tilt in any way.
    The second is a 'Nicol's prism'to be  fitted in front of the telescope to
    polarize the light from the horizon. This was simply a means for cutting out
    horizon glare.
    The third is a simple 30.00 deg prism fitted half in front of the telescope as
    a device to check centering error and  calibration of the arc. Other fixed
    angles were availiable.
    So no help here I'm afraid to help correct tilt.
    However it seems to me that if a ray splitting prism were mounted in front of
    the horizon glass, it could be used to image the horizon from two sideways
    directions, say 90 deg apart,   two superimposed horizons would now appear in
    the index glass. These images would separate as the sextant was rocked and only
    coincide if the sextant were exactly vertical. The image of the sun etc would
    then be superimposed on these merged horizons in the normal way but without the
    need to rock the sextant during the observation.
    I don't know if this is what you had in mind or if this idea has been exploited
    anywhere.
    I hope this helps your inquiry
    Clive Sutherland.
    Dr. Geoffrey Kolbe wrote:
    > There has been much discussion regarding the errors introduced by sextant
    > tilt when using a marine sextant at sea.
    >
    > There has been no discussion so far regarding the use of a "prism level" to
    > correct for sextant tilt. Does anyone have any experience with them? Do
    > they work?
    >
    > Regards,
    >
    > Geoffrey Kolbe.
    

       
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