In the clinical use of radium, it is of the utmost importance that the radium should be so distributed as to give a field of radiation as nearly uniform as possible, and of the required intensity at every point of the region under treatment. In actual practice, it is only possible to obtain a rough approximation to uniformity, but the greater the skill of the operator the closer will be his approximation and the more satisfactory the result obtained. It therefore becomes of importance to work out the distribution of radiation for certain distributions of radium of common occurence, so that this known distribution may be used as a basis for clinical practice.The exact calculation, even in the case of a single needle, involves a complicated mathematical analysis, but by a simple method the radiation at any point can be ascertained with quite sufficient accuracy for clinical purposes, even in quite complicated distributions of radium.A specially divided scale is employed so constructed that for a milligram of radium placed at any point of the scale, the reading at that point gives in r units the radiation at the origin of the scale. As a basis of calculation, it is taken that the radiation from a milligram of radium element at a distance of 1 cm. is 8 r units per hour, this being the accepted figure for a screenage of 0·5 mm. of platinum.