Our purpose in this paper is especially to stress the need for physically precise methods of measurement of X-radiation so as to render all clinical results accurate and mutually comparable. In our opinions, the presence of quantitative uncertainty in the biologic domain does not excuse the introduction of still more uncertainty in the physical domain, where precision is possible. In fact, physical precision is imperative in the measurement of the quality and quantity of X-rays given the patient before any progress in biologic precision can be hoped for. We wish also to point out a few of the pitfalls to be avoided in making physical measurements on X-radiation. The physical measurements required fall into two groups, namely, measurements of quality and measurements of quantity. We shall consider first measurements of quality. It is well known that X-rays form a spectrum of wave lengths. The radiation from an X-ray tube operated at, say, 200 K.V. constant potential is continuously distributed over a range...