In this monograph, Thoræus reports on some filtration properties of various metals checked against five qualities of radiation and illustrates in charts and tables his results. The efficiency of various metals as filters is discussed next and he presents data to assist one in selecting the best filter for a certain purpose. His new "tin filter" is described, which takes the place of 2 or 3 mm. of copper, producing hard radiation with the additional advantage of not decreasing the intensity. The monograph closes with some studies made in the water phantom on the relative rôles of true absorption and backscattering. The volume is important in helping to solve many of the puzzling problems of filtration and dosage in roentgen therapy.