Checks the perpendicularity of any flat surface to a line of sight by means of auto reflection or auto-collimation. The mirror has a circular front surface with a 2.5 inch diameter. Its reflecting surface is flat to within a ¼ wave length in 50 mm diameter. Single magnet parallel to ± 3 seconds of arc. Also available in 4″ diameter (WK-58-1894) and 6″ diameter (WK-58-1846). Three magnetic feet are cemented to the back of the 4″ and 6″ mirrors.