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    Re: accurate sextant
    From: Fred Hebard
    Date: 2008 Feb 22, 08:20 -0500

    Alex' SnoT also has luminescent paint.  The luminescence is obtained
    by exposing it to light before use, if I recall correctly.  The use
    of the magnifier with no vernier then allows one to read the sextant
    at night without a light.  Now how one writes down the numbers
    without a light is another story.
    
    The inverting scope is much lighter and slimmer than an equivalent
    binocular, if a bit longer.
    
    Fred
    
    On Feb 22, 2008, at 5:10 AM, George Huxtable wrote:
    
    > Another unusual feature is that there is a magnifier, to ease the
    > reading of
    > the micrometer drum, and also there's no Vernier with that drum
    > ( which,
    > otherwise, would be used to interpolate the minutes to tenths). I
    > suspect
    > these aspects are connected: if there was a Vernier, then the
    > magnifier
    > would have to be able to swing, to read it over its span of 10
    > divisions or
    > so, around the drum's arc. So doing without the Vernier allows the
    > magnifier
    > to be fixed in place in a simple housing. The choice made by the
    > designer,
    > then,  was either a simple magnifier or a Vernier, but not both. I
    > suggest
    > that any observer worth his salt should be quite able to estimate
    > to tenths
    > of a minute without a Vernier to aid him, especially with a
    > magnified image.
    >
    > One virtue in providing a magnifier could be in allowing the
    > diameter of the
    > read-off scale on the drum to be kept small. Otherwise, if a drum
    > is made
    > large enough to be read with ease directly, then the shaft joining
    > it to its
    > worm must be made long enough to avoid that large drum clanging on
    > the arc,
    > which increases the bulk of the complete read-out assembly. This is
    > a bit
    > speculative, as I haven't seen one of these sextants, just Bill's
    > pics.
    >
    > Finally, I ask those familiar with this instrument whether they see
    > any
    > advantage in the straight-through, inverting, 6x telescope,
    > compared with an
    > equivalent, non-inverting, prismatic ocular of similar power and
    > light-grasp? Presumably, the prismatic would be a bit heavier; are
    > there
    > other differences?
    >
    > George.
    >
    
    
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