1. Raper, H.R.: Clinical Preventive Dentistry, Ed. 1, Rochester, New York, 1926, Ritter Dental Manufacturing Company, page 46.
2. Simpson, C. 0.: Technique of Oral Radiography, Ed. 2, St. Louis, 1928, The C. V. Mosby Company , page 62.
3. Ennis, L.M.: Dental Roentgenology, Ed. 3, Philadelphia, 1939, Lea and' Febiger, page 44.