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>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.

Thankyou for the above Clive.

This is pretty much what I had in mind and the idea has been exploited.
Celestaire used to sell such a prism, which clipped onto the left hand side
of the horizon glass. They might still do, I have not looked at their
catalog recently. The claim was that verticality to 0.5 degrees was easily
achievable using this device, which cost around $40 ten years ago.

Geoffrey Kolbe.

   

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