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    Re: Perpendicularity
    From: Cliff Sojourner
    Date: 2004 Oct 12, 13:25 -0700

    is there any effect from index of refraction of the glass in
    front of the silvered surface?
    > Think of the mirror as a plane of clear glass perpendicular to the plane of
    > the instrument. Rotate the glass on the axis of the instrument until the
    > reflection of 120 degrees on the arc (as seen on the reflecting surface of the
    > glass) coincides with 0 degrees seen through the glass. You'll find, of course,
    > that the glass itself crosses the arc at 60 degrees, half way between 0 and 120.
    > That's why the system of adjustment works.
    > Now, keeping that plane of glass perpendicular to the frame of the
    > instrument, move it a couple of inches away from the axis, toward the arc. The original
    > system breaks down.

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