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    Re: Dipmeter: was [NAV-L] Wires, back sights and collimation
    From: Alexandre Eremenko
    Date: 2004 Dec 1, 13:41 -0500

    Dear George,
    On Wed, 1 Dec 2004, George Huxtable wrote:
    > Alex Eremenko wrote, referring to my use of a modified sextant as dipmeter-
    > >The only question is how one adjusts the mirrors
    > >or the prisms
    > I'm sorry, but I don't have a precise analytical answer to give to Alex.
    A theory of such periscope device is probably not hard to develop,
    but I doubt there is any need in it: you seem to be the only person
    who uses such device, and I conclude fom your messages that you do not
    worry much about its adjustment or accuracy:-)
    > The backward-looking periscope was as-bought, part of some obsolete
    > military sighting equipment, at a guess. From the robust way it is
    > constructed, the angle between the two mirrors should indeed be very
    > stable.
    I conclude that the mirrors on your device do not have adjustment screws,
    This raises an interesting general issue: why do they never fix the
    mirrors rigidly on a sextant, so that they are strictly perpendicular
    to the frame and dispense with the adjustment screws?
    Apparently it cannot be guaranteed that they remain perpendicular...
    In your message of Sun Nov 21 2004 - 12:05:39 EST
    you criticized Dollond who proposed to make some rigid non-adjustable
    "gimmick" to permit index correction with back sights:
    >The flaw in that argument, as I see it, was that it relied on the
    >exactness of that 90-degree shift, over long periods, and any mechanical
    >would result in errors for all back observations.
    I think the same criticism applies to ANY device where the angle between
    mirrors is important. This angle should be adjustable.
    (Your periscope was probably not originally intended for precise angle
    > instrument is used normal way up and inverted.
    From your sketch of the device, it is hard for me to imagine
    how you can use it upside down.
    Do you have to bend yourself so that your head is in horizontal

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