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I've been away (in New Zealand) for the last few weeks, so have missed
much interesting Navlist discussion. Back, now, and with the spam
swept out of my inbox, I can respond to some of the backlog.

=================

Alex posted the following message on
 Sunday, January 28, 2007 3:28 PM
Subject: [NavList 2136] Re: Old Sextant on German money


|
|
| On Sat, 27 Jan 2007, Robert Eno wrote:
|
| > I visited Germany about 10 years ago, and again a few years later.
Of all
| > the souvenirs I took back with me, guess which one I still have
safely
| > squirreled away?
|
| Sure. So did I. But I never seriously examined this
| picture before. The most curious part is
| the strange split index mirror which I've never seen
| before in the museums, e-bay or books.
|
| I know the Maskelyne proposal to make half of
| the index mirror silvered and half black.
| The silvered part for stars, the black part for the Sun.
| The purpose of the black part is to eliminate
| the second reflection (in a normal back silvered mirror,
| both surfaces reflect, and the front surface creates another
| much weaker image. Maskelyne's idea was to use for the Sun
| the front unsilvered surface only, because the light from
| the Sun is strong enough).
| But as I understand this Maskelyne proposal, the mirror
| should consist of one piece, only half of it
| silvered and another half blackened.
|
| In this particular sextant, we clearly see
| two separate mirrors.
| And they even do not look parallel!
|
| Also, the absence of horizon filters looks very strange.
| This is not a "cheap octant" for altitudes only;
| this is definitely a high-end sextant/pentant whose main
| purpose in XVIII century would be the lunars.
|
| Another feature I do not understand (though I saw it on
| many e-bay sextants) is a large flat disc surrounding
| the reading microscope. What is its purpose?
| Some older sextants frequently have it.
|
| Alex.
|
=================

This followed an earlier posting, Navlist 2132, on 27 Jan-

=================

Dear list members,
Many of you probably know that there is a picture of a sextant
on the famous German 10 DM bill dedicated to Gauss.
(Unfortunately, this money bill was removed from circulation with
the introduction of Euro. I saved a few of them, unfortunaly very
few, and until recently, some of these bills were traded on the
Internet, see, for example,
www.math.purdue.edu/~eremenko
and click on the "Portrait of Gauss".
There you can see the picture I am refering to in the rest of this
message.
(The face of the 10DM bill has a portrait of Gauss, together with the
graph of the Gauss Law (a.k.a. Normal Distribution, a.k.a. Bell
curve),
a very nice picture, and the correct formula).
The back of the bill has a very detailed picture of a sextant
(much better than most e-bay pictures:-)
and also a map of the triangulation Gottingen-Altona (Hamburg) that
Gauss
made.

I am mostly concerned with the sextant.
It is a double frame (=coumn frame) vernier sextant/pentant
probably of the late XVIII century. Notice: it does not have horizon
filters.
It has some strange horizon mirror adjusting device (Dollond?)
and something underneath the horizon mirror which I don't understand
what it is.

But the most curious feature of this sextant is the Index glass.
It looks like consisting of two pieces, the kind of an index glass
I've never seen before.
Can anyone answer what is this?

(There is no doubt that the artist who made this engraving had some
museum piece
in front of him, and tried to reproduce it as precisely as s/he
could).

Alex.

==========================

George comments-

List member Peter Ifland, in his book "Taking the Stars", got there
before Alex did. His figure 45 is captioned "A pillar-frame sextant by
Robinson and Barrow, ca.1830. A remarkably similar pillar sextant
appears on a 1987 series German ten-mark bank note commemorating  a
survey by triangulation across northern Germany."

And indeed, that photographed sextant is uncannily similar to the one
in the note, illustrated for us by Alex. There are very minor
differences, but clearly both follow the same basic design.

The illustration of the banknote is by no means crystal-clear, and
comparing it with Ifland's photo, I would not be so confident as Alex
is when he states- "It has some strange horizon mirror adjusting
device (Dollond?) and something underneath the horizon mirror which I
don't understand what it is." Those presumed components could well be
simply a view of the horizon shade assembly, which Alex claims to be
missing.

Alex is right in claiming- "But the most curious feature of this
sextant is the Index glass. It looks like consisting of two pieces,
the kind of an index glass I've never seen before.
Can anyone answer what is this?"

and he adds-

"(There is no doubt that the artist who made this engraving had some
museum piece in front of him, and tried to reproduce it as precisely
as s/he could)."

Perhaps not, though. That double-mirror makes no more sense to me than
it does to Alex. I wonder whether the engraver was working from a real
sextant, as Alex assumes, or was doing his best to interpret another
drawing: perhaps one that showed the index mirror in two positions to
indicate the way it moves. The engraver was unlikely to understand the
workings of what he was drawing.

Alex refers to the Maskelyne's trick of blackening the back of part of
the index mirror, to eliminate double reflections. That was indeed a
problem in Maskelyne's day, but later, when opticians learned to to
grind their glass accurately parallel, it became unimportant, as
images from two parallel glass surfaces coincide exactly.

Alex refers to the large disc which moves with the magnifier, clearly
shown in figure 45 and also in one of the sextants of figure 49. Its
purpose puzzles me too. My speculation is that it might be backed with
a white disc on the unseen side, which would reflect diffuse light on
to the Vernier from an oil-lamp or candle placed close behind the
sextant, at the same time shielding the observer's eye from that
direct light. Just a guess.

Another puzzling matter, to me, is the relevance of a hand-held
sextant to a survey triangulation on land. In hydrographic survey,
sextants can provide precise horizontal angles, but then everything is
in one defined plane, that of the sea-horizon. Not so for a land
survey, when landmarks are up hill and down dale, and some sort of
theodolite, firmly planted on the ground and precisely levelled, is
needed to give the necessary elevation angles.

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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