NavList:

Tony Oz you wrote: If yes, then it is possible to zero-out all errors. The diagonal mirror looks adjustable - just as the mirrors on a sextant itself.

I'd started without the bubble attachment - and completed the usual sextant adjustments (the index mirror perpendicularity, the side error on the horizon mirror, the index error). Then - setting the index to zero, I'd adjusted the diagonal mirror in the attachment so that the natural horizon (when the sextant's index is at the zero) cuts the bubble through the center.

If you do that Tony, wouldn’t you have effectively set the bubble attachment to a false vertical?  Wouldn’t you at the time and place of adjustment have to note your height of eye above the horizon and work out a fixed ‘dip’ value to be subtracted from the Hs every  time you subsequently used the sextant in ‘bubble’ mode?  DaveP