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George Huxtable wrote:

> I think it's accepted (isn't it?) that from the clear, unsilvered part
> of the horizon mirror some fraction of the sunlight incident on it
> from the index mirror will be reflected by the glass surface into the
> telescope. And in roughly equal amounts from the front surface and the
> back.

Full agreement.

> The argument put forward, which I wish to dispel, is that this
> would create two separate light beams, displaced vertically by some
> amount that depends on the thickness of the horizon glass, [and
> therefore to two separate images of the Sun as seen through the
> telescope]. The only part of the argument that I argue with is the
> final phrase, that I have put within square brackets.

This is where we differ, and also whether the reflections from the glass-
air surfaces play any role in normal sextant sights.

I'm afraid I'm rushed right now but I promise to consider your message
more fully later on. Two things right now: Last night I was observing
reflections from the glass part of the horizon mirror using my eye with the
scope removed. I had to do this in a dim room to see the reflections
clearly, but with my eye located in the normal line of sight (i.e. from
where they eyepiece would be if the telescope was there) I always saw a
double reflection. These were not reflections from the index mirror as I
was unable to see those in the dark room. Also, I tried looking through
the scope at a white wall with a bright ceiling lamp being brought down
from the index mirror onto the wall (this time with the scope). Normally, I
see a full-horizon view of the lamp. When I block the horizon mirror (but
not the glass) with a piece of paper I see no lamp at all. There was the
faintest hint of a highlight, but no detail whatsoever. If I block the horizon
glass with paper (leaving the mirror open), then I see a full horizon image
of the lamp.

One more thing, regarding my anecdote about the porch light going on
behind me. I repeated it last night and found that from where I was
standing, the light itself was reflected in the horizon glass, my face was
partly reflected in the horizon shades below the glass. When the light
went on I immediately lowered the sexant and looked into the horizon
glass from above the telescope. I probably did the same thing when this
happened by accident a few days ago and this last image was the one I
remembered. You are correct that this anecdote is irrelevant.

Ken Muldrew.

   

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