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I was waiting for someone else to weigh in on this.  Since Bill was
not positive about the fluid, I might suggest a silicone fluid.  This
is used on the older C. Plath bubble horizons that are adjustable.  I
think there is more danger of xylene interacting with rubber-like
parts that may be in there.  I have filled many C. Plath chambers by
just spraying a silicone aerosol into a glass until it fills with
enough fluid to inject into the chamber.

Ken Gebhart
On Apr 6, 2009, at 6:49 PM,  wrote:

> Attached is a sectional drawing of the bubble unit of a Soviet IMS3
> sextant. The instrument has many similarities to the SOLD, as the
> latter was made by C Plath, whose satellite factories were over run
> by the Soviet Army towards the end of WWII. The bubble units
> appear, at least outwardly, to be identical.
>
> 114 in the diagram appears to be the screw that, with washer 115,
> closes the filling hole. The fluid was probably xylene or hexane,
> rather than the alcohol solution or filtered kerosene used to fill
> compasses. Xylene (or hexane in the UK Mk IX series) was chosen
> because its refractive index is close to that of glass and so the
> upper "glass" of the unit would not act as negative lens when the
> chamber was filled.
>
> The drawing has five pages of cyclostyled text in Russian referring
> to it. I can scan and send these to anyone to whom they might be of
> use or post them on my blog. Let me know what you would like.
> >
> 


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