Re ceiling dials, Lets get empirical:
I. Take a piece of ordinary looking glass and hold it by its lateral edges, close to and beneath, your eye, and look at some test grid (e.g. the day squares on a calendar) across the room. Compare the appearance of the squares' lines by low-incidence reflection from the mirror, with direct view by the same eye. Are the horizontal lines of the top and bottom of the squares more -- or less -- distinct than the vertical lines of the squares' sides? If there is a difference between the clarity of the vertical and the horizontal lines, this is an indication of astigmatism. If it occurs only in the low incidence mirror reflection, the mirror is not flat. If it occurs also in the direct view, your eye is then its source. If it occurs not at all, or only slightly in either or both cases, you can probably use that mirror for a reflected dial, since the incidence angles for the working dial will be larger than for this test, which is a surprisingly sensitive one. Try the same test on a piece of window glass. (If you wish, you can later make it into a front-surface-only mirror by "grinding" the back side on a piece of carborundum paper, and coating it with black paint.) Question: If the effect on a narrow grid line is so small an angle across the room, how much difference will it make in reading the time on a ceiling dial? Place patterns of masking tape (cross lines or apertures) on these mirrors, and see if they help to define a reading-point on the ceiling. If they do, you can paint more lasting patterns directly on the glass. (You know, just like when painting the trim around your house windows.) II. Put a little water in a shallow dish (e.g. an inverted plastic "snap lid" from a small jar or cannister) and place it on a sunlit window sill or horizontal table-surface. Note the visibility of the reflected light spot. Repeat the experiment with some cooking oil in place of the water. Do a "river dance" beside the window. Observe any effects on the light spot for each liquid. (For a more permanent device, mineral oil will be less perishable and available with higher viscosity, for better damping. But you'll still need a transparent cover to keep dust from settling on the surface. While you're playing with these liquid mirrors, note that miniscus effects have little importance for one's ability to "read" the spots in practice.) Question: Do you really need mercury to make a self- leveling mirror? Sciagraphically, Bill Maddux ------------------------------------------------ [The mercury paraboloid experiments "decades ago" were the work of Dr. R. W. Wood, Professor of Experimental Physics at Johns Hopkins, who in 1908 - 1909 used a 20 inch diam. rotating shallow pan of Hg in an attempt to produce an optical mirror.]
