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Dear Bruce,

I carefully read your explanation and I think I understood it.
But it does not answer my question.

Could you please do the following experiment with your sextant
for me? (Not in your mind, but on your table).

1. Place the sextant on the table, handle down, arc horizontal,
index mirror toward you, shades out.
2. Put the index arm as you said, on 35 d.
3. Put your eye at a distance approx 1 foot from the index mirror,
roughly in the plane of the arc, but slightly above this plane,
so that you see the arc.
Use good light to illuminate the arc, and be sure that
it is seen against a contrast background.
4. Move you eye slightly so that you see a part of the arc
directly and another part reflected in the index mirror.
Exactly as you described, and as every book describes.
5. Suppose your sextant is adjusted and the reflected image
of the arc seems to be continuation of the part of the arc
seen directly. These two arcs seem adjacent near the right edge
of the index mirror. Let X be the point in this right edge where
the images of these two arcs come together.
6. Move your eye slightly up and down, without loosing the sight
of these two arcs. You see the point X moving along the right edge
of the index mirror up  and down, in the same direction as your eye.

Do you see this?

7. Now notice that the arcs do NOT stay aligned as you move your eye
up and down.
They can be aligned only for ONE position of your eye and the point
X.

Is this what you see?

I see it with my sextant, and simple geometry also shows that this
should be the case, unless the index mirror is very small.

My question is: what is the proper position of the point X
(or the height of your eye over the plane of the arc) in the
proper test?

On Mon, 11 Oct 2004, Bruce Stark wrote:

> Alex,
>
> This isn't an easy thing to see in one's own mind,
> and I doubt I could
> explain it clearly. Sometimes it's easier to just do something,
> then figure out for
> yourself why it works later on.

   

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