NavList:

There was another method of attempting to improve accuracy by using a prism splitter doubling device. These kind of lenses/prisms are used occasionally in photography to give multiple images for special effects.

Instead of seeing one star - you see two - two of everything in fact. I am not sure if this was expected to improve accuracy if used conventionally having two stars on a horizon: but might have been an attempt to improve the appearance with the bubble, straddling it with two images of the star on each side to give better centring. I have not tried it yet as the bubble level is dysfunctional, needs refurbishment and I have not started yet to repair it.

Here are some pictures of the beam-splitter device from my 1930s Plath 'System Admiral Gago Coutinho' sextant, which incidentally was the first successful sextant for aircraft use with bubble reference available and used in a flight in 1927 from Lisbon to Rio de Janeiro with Captain Jorge Castilho as navigator.

The first is a picture of the splitter prism device.
The second a picture taken through it outside the window.
The third looking through it at a laser dot on the wall opposite.

Douglas Denny.
Chichester. England.
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