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    Re: mirror horizon
    From: Patrick Goold
    Date: 2011 Mar 24, 22:43 -0400
    Bill and Fred,
    I started out with three levelers, thinking a tripod would form a steadier base and be easier to level.   In the event, it seemed the assembly was too unstable with only three levelers.  It tended to move when touching any one of the bolts.   And it was impossible to level in any systematic way.   Then I recalled my training as a surveyor's assistant many years ago.  A theodolite has four levelers and there is a set order for adjusting them.   The introduction of a fourth leveler has steadied the assembly and does seem to be easier to level, using the standard routine.  I should have used a bolt with a finer thread, however.  24tpi is still too course.   The level is so sensitive that a thirty-second of a turn is too much at times.  Final adjustment involves touching the bolt and then just imagining that one is turning it.

    Bill, I can't quite see what the coin and washer arrangement looks like.  Could you draw me a picture?

    Alas, there will be no tests for the next three or four days.  A cold front brings continuous cloud cover.


    On Wed, Mar 23, 2011 at 10:02 PM, Bill Morris <engineer@clear.net.nz> wrote:


    Use only three screws, two along one side and the other in the middle of the opposite side. Place the level on the mirror parallel to the line of the first two screws and level. Then place it at right angles to that line and level with the third screw. Repeat as a check. Provided the pitch of the screws is reasonably fine, this should take no more than two minutes. However, if you take it outside and set it up on a rough concrete surface, rotating the screws will tend to make your breadboard move about, affecting the level, so as an improvisation, I suggest you Araldite three small white metal American coins to your chosen site and surround them with large washers that are rather thicker than the coins and just fit around them.

    A layer of felt between the mirror and the board would be a good idea to take up any irregularities, and make sure that there is free movement of the mirror on the board. The transparent pieces of plastic you are using should not press directly on the mirror, i.e. use them to locate but not to fasten the mirror, or you will introduce distortions.

    Bill (noone-calls-me-William) Morris
    New Zealand

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    Dr. Patrick Goold
    Department of Philosophy
    Virginia Wesleyan College
    Norfolk, VA 23502
    757 455 3357

    Charles Olson: "Love the World -- and stay inside it."

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