A magnetic mirror is used to check the perpendicularity of any flat surface to a line of sight using the optical principles of autoreflection or autocollimation. The 6202 mirror has a circular front surface with a 2½" (63.5 mm) diameter. Its reflecting surface is flat to within ¼ wavelength of light over any area not greater than 50 mm in diameter. A single magnet is cemented to the back of the mirror. Its contact surface is ground parallel to the mirror surface within 3 arcseconds.  This type of mirror is also a convenient accessory for performing instrument calibrations in the field. 

Below is the back side of the 6202 showing the magnetic mount.