- Telescope mirror which might be created by spinning a pool of mercury (the metal) to produce an optical surface capable of forming an image. The notion is over a century old and has the attraction that the ideal shape for a telescope mirror, a paraboloid, is assumed automatically by a spinning liquid surface. Using modern methods – including smooth electric motors to guarantee a good image – it might be possible to build cheap liquid mirrors much larger than practicable solid ones. Problems would include the toxicity of mercury. A 2.7m test mirror at the University of British Columbia in Canada has shown that the principle works and can provide a cheap mirror capable of producing good images. But it only does so while the telescope points at the zenith: tilt it and the parabola is distorted by gravitation.