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Geoffrey Kolbe wrote-:

>>At the point where you have set aside the welder's glass, and swung in the
>>dense shade in the index-mirror view, you still have the full brilliance of
>>the Sun in the horizon-mirror view. Unless you introduce a similarly dense
>>shade into that direct view, your eye will be quite unable to take it. But
>>if you do that, you are measuring, not the error of the upper shade, but
>>the combined error of two shades (just as Lecky described), which was NOT
>>the object of the exercise.
>
>George, are you able to remove the index mirror shade and rotate it  - even
>using plasticene or some other jury rig to hold it in place while you see
>if the IC has shifted....?

Yes, that was rather what I had in mind when I wrote yesterday-

"Perhaps the best way for me to assess the error of that darkest shade might
be to detach it from its normal mounting (which is rather easy) and instead
cobble it back into roughly the same spot using insulting tape, in such a
way that it can be inverted, top to bottom. If that shade is indeed found
to be prismatic, it could then be oriented to such an angle  that it gave
rise only to a small sideways displacement of the image, and didn't affect
sextant readings."

George.

===============================================================
Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
or from within UK 01865 820222.
Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
5HX, UK.

   

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