John Carmichael suggested floating the mirror in mercury to level it, and then Dave Bell pointed out that the clean surface of the mercury could be the mirror.
Since mercury is a bit of a problem to work with, could a dish of water serve instead? In John's case, he just needs the mirror to be able to float on the liquid. I just tested a CD and it floated on water. The plastic is more dense than water but if the CD is dropped gently into place, it floats due to surface effects. A small piece could be cut from a spare CD to make a mirror of the required size, or the hole at the centre of the annular reflection of an intact CD could act as the spot. A CD is not as reflective as a mirror of course, but I think it would be good enough. I can't test this - it's cloudy here today. The dish would need to be only slightly wider than the mirror to prevent it floating off centre. Also, the dish might need to be full to the brim so that the sunray does not need to travel through its the walls. Alternatively, could the reflection from water itself be enough, like Dave's way with mercury? A vessel such as a wineglass on the window sill would look quite welcoming from outside. Of course, the choice of liquid would be important, and contrary to the sundialling norm, Moonshine might work better than a Tequila Sunrise. Steve
