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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
> > | >
> ____________________________________________________________
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