1. A brief history of endodontics;Gensman;J Endod,1982
2. http://compepid.tuskegee.edu/syllabi/clinical/small/radiology/chapter2.html
3. http://www-history.mcs.st-and.ac.uk/Biographies/Morgan_William.html
4. Radiographic simulation of a periapical lesion comparing the paralleling and the bisecting-angle techniques;Forsberg;Int Endod J,1994