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Hello David  I have observed both horizons via a prismatic device that the Soviets developed and as well with my circle of reflection.  In both devices, you observe the horizons which are 180° apart in azimuth.  In practice, horizon A is aligned with horizon B.  What is observed is the dip of the horizon for both, when summed.  Consequently, the observer is unaware of the contribution of each towards the sum.  The sum is divided by two, to obtain the dip by measurement instead of by tabular lookup.  To preset your device, you might wish to set the expected dip via table using the height of eye as the independent variablr, but in practice, I just start at 180° and increase the setting while observing until the horizons are coincident.  So in answer to your Q3, I would state: zero, the difference observed between horizons should be zero when the observation is correct. Brad

Thanks Brad.  That's what I thought.  The reading would be posititive and you'd have to divide it by two to get the mean dip.  Perhaps I didn't phrase Q3 very well. The point I was trying to make is, for the system shown in the photograph, the multi reflected horizon would be upside down, because of the arrangement of the prisms.  For a backwards looking periscope, after two reflections, the highest ray comes out lowest, c.f. a forwards looking periscope where it comes out highest.  DaveP