NavList:

Bob I hope will not mind if I make one or two corrections to his post about sextant bubble units.

In some sextants where the light path from the observed object passes through the bubble chamber, the refractive index of the fluid is probably important, as the bubble fluid then forms part of the refracting system. Thus, ethanol, water, ether, acetone and hexane are unsuitable replacements, as their refractive indices are all around 1.35, versus 1.5 for crown and 1.6 for flint glasses. The Mark IV, AN 5854-1, AN5851-1, A4, and A7 all had bubble chambers of this type and were originally filled with xylene, which has a refractive index of very nearly 1.5. Ideally, they should be refilled with xylene or a silicone oil of refractive index of 1.5. Bubble sextants in which the bubble chamber does not form part of the light path of the observed object, but in which an image of the bubble is inserted into the light path by a partially reflecting beam splitter, usually in the form of plain glass, are the MK IX series, the German SOLD, A6, A8, A10, A12 and the Kollsman periscopic sextant and its derivatives.

The Mark IX series chambers were originally filled with n-hexane and the SOLD with a nasty substance which I think was probably di-methyl sulphoxide, which incidentally has a refractive index of about 1.5. The Soviet derivative of the SOLD was filled with alcohol which is kinder to the skn and nose and the refractive index of the fluid does not seem to matter. The A 6 to A12 were filled with xylene, but can be filled with hexane or even cigarette lighter fluid (naphtha) the refractive index of which is unimportant but is around 1.44. Practically every bubble chamber that I have come across that was filled with xylene has leaked, whereas many Mark IX chambers, sealed with shellac and filled with hexane, are still fuly functional after more than 70 years. 

What you use to refill your bubble chamber with depends on how you choose to re-seal the glasses. If you choose to re-seal with melted flake shellac and it is feasible to do so (see, e.g. http://sextantbook.com/2008/12/), then you should choose an aromatic hydrocarbon such as hexane, xylene or naphtha, in which shellac is insoluble. You should not choose alcohol, as this will dissolve the shellac. A simpler alternative is to use the thinest Viton O rings you can find. Viton is resistant to the hydrocarbons I have mentioned, unlike  neoprene, from which ordinary O rings are made. Some bubble chambers were sealed with lead washers or a white plastic that I guess was high density polyethylene. In my experience, they invariably leak if re-used and are better replaced with Viton O rings or shellac. I have had no problems re-filling A12 chambers with alcohol.

Those bubble sextants which used radium paint to light the bubble can be resurrected (see the third post at http://sextantbook.com/2008/12/). In brief, the old paint is dissolved in acetone and disposed of thoughtfully. It is replaced on the bottom glass by a rim of photo-luminescent paint ("luminous paint") and elsewhere by a layer of white paint with photo-luminescent paint on top. Before use, you shine a bright light through the eye piece for a minute or so to charge up the paint. I find a small pocket torch with an ultra-bright LED ideal.

Bill Morris

Pukenui

New Zealand