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George Huxtable wrote:

> That [my 2nd paragraph] refers to the two-cylinders method of alignment, doesn't
> it, not the
> method where you look at the reflected edge of the arc, which is what I was
> considering. Or have I missed something?

Yes, but further down I deal with the arc, too. It's really the same problem. What
happens if you have a pair of cylinders that are a little shorter than ideal? Then
you bring your eye into the plane parallel to the limb passing through these
cylinders. And if the cylinders are of height zero? Well, then you won't be able to
do this, because you won't see the "top of the cylinders", i.e. the arc itself. You
have to cheat and look from above.

So we have 3 cases.

1. The mirror plane is centred on the pivot. Any method works fine, because you can
look from any vertical angle without getting parallax.

2. The mirror is to the side.

2.a. Using cylinders, look from correct position in proper plane and get no
parallax.

2.b. No cylinders available, must use arc, tough luck, parallax unavoidable.

Herbert Prinz

   

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