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    Re: Double vial horizon prototype
    From: Bill Morris
    Date: 2012 Jan 10, 15:12 -0800


    I don't think it would be wise to glue the mirror to the levelling plate. As the glue cures, the mirror may take on the contour of the plate. A simple way of retaining the mirror would be to use three pan-head screws, tapped into the plate just outside the periphery of the mirror, and tightened so the heads are just short of touching the mirror. This will prevent you carelessly decanting the mirror on to the floor, but will not introduce any strains into the glass.

    Grinding two plates together may well give you a surface that is smooth, but it is very unlikely to be flat to the standard required. A 10 second level with feet 100 mm apart will comfortably detect a difference in level of .005 mm over this distance. If you grind two circular plates together, more likely than not one will be concave and the other convex to match. This is quite desirable when mounting drums of marble on top of each other, but not when you want a truly flat surface. I am inclined to doubt, however, that the technology of the times of the Parthenon, was able to produce steel plates of the required size or even at all.

    I suggest you have a friend face the plate in his lathe. A properly adjusted lathe will face very slightly concave, so the mirror will be well supported at its periphery, which is where the feet of your level will rest when setting level. I don't think it's a good idea to divorce the level from the surface that you wish to level, and suggest you stick to resting the level on the mirror face.

    Bill Morris
    New Zealand

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