Hi John, I agree that some leveling surface like your idea of a mercury pool with the mirror on it would easily give a very level mirror for the traditional ceiling dial.
Again, using a container large enough that the area for the mirror is free of meniscus is a good idea. Since we were originally thinking of this design for a classroom of young students, mercury bothered me. I tried curing a small amount of two part room temperature curing casting resin in a jar lid a bit larger than my mirror and then resting the mirror on this surface. As far as I can tell it is quite level. I don't know the brand name of this stuff, it was given to me as part of a larger supply a few years back. I did stir it from the bottom to eliminate bubbles with a bent wire stirrer. Thanks for the idea! Edley.
