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Brian  Villmoare you asked about A12 bubble chamber fluid.

Brian

I think there might be two possibilities here.  One is the liveliness of the bubble, which of course depends upon the viscosity of the fluid.  The other is what you might call “tiltproofness” (Brits can invent words too).  I.e., if you tilt the sextant forwards or backwards, does the star remains in the centre of the bubble as both move.  For this to happen the radius of the top of the inside of the bubble chamber has to equal the focal length of the collimating lens (Louis Fave 1906, Booth and Smith RAE  1919 ) for reasons I’m happy to call ‘magic’.  Now, does the reflective index of the fluid alter the focus point of the upper lens of the bubble chamber?  I don’t know, but I’m sure someone on this forum does.  Also, does a slight displacement of the top glass of the bubble chamber on reconstruction affect things or perhaps over-cleaning, which might alter the radius of curvature?

I find the biggest problem with a refilled A12 bubble chamber is evaporation, despite my care, which causes the bubble to grow.  In the end, I broke all the rules, filled mine with white spirit, which doesn’t evaporate so quickly, and it works fine enough for my sort of sextant play except it's not very tilt proof.

Finally, always work on an A12 bubble chamber over a tray just in case the tiny sealing ball bearing falls out.  If you’ve overdone the varnish and the ball is stuck in, the cheapest acetone nail varnish remover will free it. DaveP