1. Research Paper RP612;Kessler;Journal of Research of National Bureau of Standards,1933
2. Manufactured by the Sherman W. Frazier, Washington, D. C.
3. Manufactured by the Taber Instrument Corporation, North Tonawanda, N. Y.
4. The S 11 refacing disks were supplied by the Taber Instrument Corporation.
5. Since this article was written, the radius of the path has been increased to 45 mm.