# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Logic of shades
From: Francis Upchurch
Date: 2018 Oct 27, 07:43 +0100

```Alex,
Re slide rules,
My homemade cylindrical Fuller can do 4-7 decimal places and I have excellent
results in CN. My Bygrave  gives very good results for Alts and great circle
distances but is not a general use slide rule like the Fuller.
I can provide more details on request.
Francis

-----Original Message-----
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of alexander duytschaever
Sent: 26 October 2018 17:16
To: francisupchurch---.com
Subject: [NavList] Logic of shades

Hello,
I'm a n00b user/amateur astronav that dived into astro navigation after
stumbling over a Bris sextant and wondering if one could ever perform LOP
calculatons using a slide rule (got a few of those) :-) (*) So I soon
acquired a small shiny King Hughes 1917 replica and recently a Davis Mk 15,
and I can happily live with their shortcomings (found an article on how to
deal with those shortcomings, and I still have to calibrate these three).
My question is: what is the logic behind the different shades? I understand
that one needs more shades for the index mirror (4 vs 3 for the horizon), but
why were those colours (blue, blue, orange, grey) chosen? I suspect that the
light grey colour can be used for moderate lighting or to increase contrast,
and it also reminds of lunar light conditions; but the two blue glasses look
similar (I guess a main function is to block out UV), but are there intended
ways to combine those glasses?
thx
-alex-
(*) for now, my answer is that a SR can serve for interpolations; it doesn't
have enough digits to provide sufficient accuracy. or maybe with A LOT of
sightings, who knows...

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