A disk of high-grade quartz glass approximately 2 cm (0.75 in.) thick, having at least one side ground and polished with a deviation in flatness usually not exceeding 0.000002 in. (50 nanometers) all over, and a surface quality of 5 microfinish or less. When two surfaces of this quality are placed lightly together so that the air is not wrung out from between them, they are separated by a film of air and actually touch at only one point. This point is the vertex of a wedge of air separating the two pieces.