NavList:

Tubular spirit-Ievel artificial horizons, whether straight or curved, have
several serious limitations. Two tubes are required - one parallel to the
plane of the instrument to establish the horizontal and one at right angles
to deal with tilt of the plane of the instrument. ln the early designs, the
observer had to look at three places at once: at the reflected image of each
of the two bubbles and at the image of the celestial body reflecod in the
horizon glass. A special lens was required to keep the eye focused on the
bubbles at a few inches from the eye and at the same time, focused on the
celestial body at infinio distance. Despite these disadvantages, Admiral
Richard Byrd used a tubular spirit-Ievel artificial horizon mounted on a
conventional sextant to establish his position in his flight over the North
Pole in 1926. Brandis & Sons of Brooklyn produced a few of these
instruments, but they were never widely successful.

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"Several serious limitations" might be an overly polite way of describing the operation, from the way it was described, looking at several things, at the same time, with one eye. That the thing worked at all, seemingly it did, strikes me as kind of amazing, alas man, for or with all his faults, turns out to be a fairly clever type, sometimes.

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