NavList:

   

Reply

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.



--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply