This book sets forth the authors' views on filtered and unfiltered dosage by roentgen rays, and gives a bird's-eye view of the field of usefulness for roentgen-ray therapy. Such information has been in great demand since the use of the rays has been so widespread. The quantities given for filtered roentgen-ray dosage assume the theory that at twice the skin-target distance the therapeutic effect is one-half as great. This is contrary to the statements of other writers and is contrary to the accepted laws for radiant energy which state that the quantity of rays varies inversely as the square of the distance. Readers who are accepting the quantities set down for filtered dosage should be aware of this departure from the usual computations and measurements.