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    Re: Perpendicularity
    From: Bruce Stark
    Date: 2004 Oct 12, 14:55 EDT

    You wrote:
    "Two correspondents (Doug Royer, Gary Harkins), have informed us that the
    simple test for perpendicularity of the index mirror of a sextant (by
    checking that the arc aligns with its reflection) will work only if the
    mirror is back-silvered."

    And: "
    So I ask: is that claim actually true? Has anyone tried it out with a
    front-silvered mirror?"

    I've tried it a number of times, George, and it's pretty much as Alex says. If the reflecting surface of the mirror isn't on the axis of the instrument, your eye has to be in the same plane as the arc (or the vanes set on the arc) to check the perpendicularity. But since the front surface of the mirror is pretty close to the axis, eye position doesn't have to be precise.

    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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