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    Re: Advice concerning sextants
    From: Hewitt Schlereth
    Date: 2010 Feb 23, 20:08 -0400

    Ken -
    
    In 1980 I taught one-week celestial courses to two groups of six
    people. Each student was supplied with a Davis Mark 25 with the
    full-horizon mirror. We got uniformly good results, morning, day and
    evening; sun, moon, stars and planets.
    
    After this experience with the Davis 25's, I got a full-horizon mirror
    for my C. Plath and never switched back to the traditional one. It
    worked fine for me in all conditions.
    
    Hewitt
    
    
    
    On 2/23/10, Ken Gebhart  wrote:
    > George,
    >  I do not agree that the total must equal 100%.  Neglecting absorption, I
    > agree that light going through the mirror in one direction would have
    > reflection and transmission adding up to 100%.  But where light goes through
    > in both directions, with the proper coating on the glass, the sum could be
    > greater than 100%.  I believe that was the whole idea of the invention of
    > whole horizon type mirrors (Allview, Transflex, etc.) Am I wrong?
    >
    >  Does anyone else care to jump in on this?
    >
    >  Ken
    >
    >  On Feb 23, 2010, at 1:42 PM, George Huxtable wrote:
    >
    >
    > > Ken Gebhart wrote-
    > >
    > > "...2.  Whole horizon mirrors do not divide the light.  They have a
    > > specialized coating which transmits a large percentage of light, while at
    > > the same time reflects a large percentage of light.
    > >
    > > I don't fully understand what Ken's saying here, but whatever it is, I'm
    > > inclined to disagree.
    > >
    > > Such a mirror can reflect some light, transmit some light, and absorb some
    > > light. The total has to add up to 100% of the incident light. It can't
    > > create light that wasn't there. So it DOES divide the light, as best it
    > can,
    > > between reflecting and transmitting, absorbing as little as possible. What
    > > else is Ken telling us?
    > >
    > > The most pithy description of whole-horizon mirrors is that they make easy
    > > sights easier, and difficult sights harder. But I've had very little
    > > personal experience with such mirrors, so am not competent to express an
    > > opinion.
    > >
    > > George.
    > >
    > > contact George Huxtable, at  george@hux.me.uk
    > > or at +44 1865 820222 (from UK, 01865 820222)
    > > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    > > ----- Original Message -----
    > > From: "Ken Gebhart" 
    > > To: 
    > > Sent: Tuesday, February 23, 2010 5:15 PM
    > > Subject: [NavList] Re: Advice concerning sextants
    > >
    > >
    > > |I would like to make two comments about Jim's posts:
    > > | 1.  Bigger mirrors do not give more light. (Is your image while
    > > | shaving with a large bathroom mirror brighter than with a small one?)
    > > | 2.  Whole horizon mirrors do not divide the light.  They have a
    > > | specialized coating which transmits a large percentage of light,
    > > | while at the same time reflects a large percentage of light.
    > > |
    > > | Ken
    > > | On Feb 22, 2010, at 11:40 PM, James N Wilson wrote:
    > > |
    > > | > John reminds me of my first experience with the whole horizon sextant
    > > | > mirror, which I called the half silvered mirror, was that finding the
    > > | > horizon in dim light was difficult. I found no advantages, in that the
    > > | > sun and moon images can be seen in the clear part of the mirror,
    > > | > aiding
    > > | > in bringing them down.
    > > | >
    > > | > Now, if someone would make a horizon mirror that was half silvered on
    > > | > only the left side, that might be worth something. But I might
    > > | > still have
    > > | > the same problem at twilight, where I need all the light available
    > > | > to see
    > > | > the horizon.
    > > | >
    > > | > I'm sure that the article mentioned noted that half silvered
    > > | > mirrors (one
    > > | > way glass) allow only a portion of the light to pass. When we
    > > | > remodeled
    > > | > our bath, we had one installed in our shower for privacy. The room was
    > > | > significantly darkened, and plants wouldn't grow. The amount of light
    > > | > reduction is significant.
    > > | >
    > > | > Jim Wilson
    > > | >
    > ____________________________________________________________
    > > | > Nutrition
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    > > | >
    > cp=NVIXY2oLGPGa1TCho6UV4QAAJ1D3lHlZVltl3Pnu3pZYfILRAAYAAAAAAAAAAAAAAAA
    > > | > AAADNAAAAAAAAAAAAAAAAAAASQwAAAAA=
    > > | >
    > > | >
    > > |
    > > |
    > > |
    > > |
    > >
    > >
    > >
    > >
    >
    >
    >
    >
    >
    
    
    
    

       
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