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Commenting on the dialogue between Bill and me, developing Paul
Hirose's suggestion about a striped index-alignment board, Jim Martin
has written-

         According to "Nicholls concise guide"vol.1,a close by object
| or straight line should not be used to determin index error,because
it
| does not allow for sextant parallax and atmospheric refraction.I can
| give the textbook explanation if it is of interest.

But I think he has missed the point. What we were discussing, though
perhaps the context did not make it completely clear, was a way of
establishing what was the vertical offset, between the two viewlines
of a sextant, and then setting up some target specially marked to
compensate for that offset. That vertical offset between the viewlines
is the basis of the sextant parallax that Jim refers to. If it's
measured, for a particular sextant, and then allowed for in the
marking of the target, then it no longer becomes a problem, and the
warning in Nicholls, though reasonable as a general rule, no longer
applies.

As for refraction, the difference in refraction of two light rays,
separated vertically by only a couple of inches, and only a few tens
of metres long, would presumably be entirely negligible.

But if Jim thinks that I have got things wrong, I hope he will explain
further.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.



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