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In reply to my comments about a split-horizon mirror, as follows-

>> The reflection from the silvering is getting on for 100%. But even an
>> unsilvered glass surface reflects light to some extent, just as you
>> can see in a window-pane. Light can be reflected in this way from both
>> surfaces of the unsilvered part of the horizon mirror. So you still
>> see an image of the Sun in that part of the glass, but a significantly
>> dimmer one than in the silvering.

Ken Meldrew replied-

>This is hard to believe. Surely the front surface reflections would play
>havoc with the image if they were significant.

Why so, Ken? As long as the front and back surfaces are plane-parallel then
reflections in them will be precisely aligned with each other, and quite
undistinguishable.

>Isn't the phenomenon that
>Jim's talking about simply due to the fact that he's using a telescope?
>Sincd the telescope is focussed at infinity and the horizon mirror is right
>in front of it, the lens will gather light from off-parallel rays coming from
>behind the obstruction. These can then be refracted back into the field of
>view. Although this part of the image will be dimmer, it's still there. For
>example, if I stick my finger in front of my telescope (or binoculars,
>closing one eye so that it's a monocular), I can see the whole scene as if
>I'm looking through my finger. The part of the image that should be
>blocked is just dimmed, not extinguished. The greater the magnifying
>power of the telescope, the less the depth of field, so the further removed
>the horizon mirror is from the focus. With a sight tube (maximum depth of
>field), the mirror presents a more-or-less complete blockage because the
>eye can keep it in relatively good focus while still looking at infinity. With
>a 6x telescope, it will look like the mirror isn't there.

I ask Ken to explain how Jim could possibly see an image of the Sun that
way, looking AROUND the horizon mirror. Any such extraneous light-path
around the horizon mirror would give him a view of the horizon only.

So my explanation still stands, even if Ken Meldrew finds it hard to believe.

George.

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contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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