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    Re: Logic of shades
    From: Francis Upchurch
    Date: 2018 Oct 27, 07:35 +0100

    Welcome aboard!
    I think visual acuity varies with colour (i.e. wavelength of light). The 
    colour sensitive cone cells are more sensitive to green than to blue I 
    believe. Red is poorly perceived in low light. I am not sure if this is any 
    basis for the logic of shades.
    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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    View and reply to this message: 
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