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    Re: Bubble sextants on e-bay
    From: Geoffrey Kolbe
    Date: 2007 Jan 02, 08:56 +0000

    Robert Eno wrote [NavList 1946]
    >I wonder if you would mind describing the Link A-12 illumination system for
    Thanks for pointing out the interesting subtlety of the Mk IX bubble
    illumination system, so that the centre of the bubble is not
    illuminated. That had escaped me.
    As Paul Hirose pointed out, the A-12 does not have this
    sophistication. There is just a bulb in front of the translucent
    window of the bubble chamber. The brightness of the bulb is adjusted
    with a rheostat. While on the subject, it is worth pointing out that
    it seems to be important that the bulb has just a straight piece of
    wire as a filament, not a coiled wire filament as in most bulbs. Only
    the bulbs with the straight piece of wire seem to vary evenly in
    intensity as you turn the rheostat.
    Despite the fact that the centre of the bubble is illuminated on the
    A-12, I have had no trouble in shooting Polaris, which is a 2nd magnitude star.
    Geoffrey Kolbe
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