Histologic Phenomena of Tooth Tissues as Observed under Polarized Light; with a note on the Roentgen-Ray Spectra of Enamel and Dentin**From the research laboratory of the dental school of the Ohio State University, Paul C. Kitchin, director.Read before the Research Session at the Sixth Annual Meeting of the American Association of Dental Schools, Chicago, Ill., March 26, 1929.