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On 4/04/2014 3:11, Brad Morris wrote:
> Anon, like me, can see no use for the up arrangement of the pentaprism.
> I still don't understand why Nicolas would do that.  He's a bright
> fellow, I'm sure he had a reason.

It's a surveying sextant, meant to be used horizontally.

If a sextant's normal range is -5° to 125°, the range with the
pentaprism 'pointing down' will be 85° to 215°, so if the prism were
fixed you wouldn't be able to measure angles smaller than 85° without
unscrewing the prism housing from the frame.

With the pentaprism in the other position the range will be -95° to
35°, returning the ability to measure angles smaller than 85°.

Both the C&P and Freiberger pentaprism attachments can be rotated 90°,
of course:

http://www.cassens-plath.de/katalog/index.html?startpage=90
http://www.fpm.de/index.php?c=1&s=pentaprisma

Luc

   

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