A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Tony Oz
Date: 2021 Sep 17, 09:09 -0700
does it make a real difference?
In my experience (with the SOLD KM2 Soviet clone - ИМС-3) it only matters if the chamber should be transparent to let the natural horizon to be seen. In this case there will be slight de-focus on the natural horizon because of optical density mismatch (there is a lens between the natural horizon and the chamber). I tested my ИМС-3 to check for the index error:
- through the properly-filled chamber
- through the dry chamber
- without a chamber
The second to last case gave a rather blurred view, the last one gave severe defocus.
If the bubble is to be seen alone - my strong opinion is: it does not matter much which liquid is used. It only matters the viscosity and temperature coefficients to make the bubble freely movable. I tend to think the xilene was chosen to prevent the fluid from being drinken by servicing personnel. The "same as the glass optical density" story was invented to not tell the truth - the top brass was sure - if any kind of spirit was used - it WOULD be "consumed".
That's my two-pence.