. The American journal of roentgenology, radium therapy and nuclear medicine . horizontal distances de-note distance from applicator to skin,the vertical distances give the intensityof radiation from the given applicator, inpercentage of the intensity from a pointsource of the same strength at the samedistance. Since the effect due to the pointsource is easily calculated by the inversesquare law, it will be seen that when wehave obtained a relation between a givenapplicator and the point source, we canreadily find the effect due to the any given applicator at a definite dis-tanc
. The American journal of roentgenology, radium therapy and nuclear medicine . horizontal distances de-note distance from applicator to skin,the vertical distances give the intensityof radiation from the given applicator, inpercentage of the intensity from a pointsource of the same strength at the samedistance. Since the effect due to the pointsource is easily calculated by the inversesquare law, it will be seen that when wehave obtained a relation between a givenapplicator and the point source, we canreadily find the effect due to the any given applicator at a definite dis-tance, it is necessary to express the distance 8 A bare applicator is one having no additional filter be-yond the glass of the emanation tube. in terms of a for that applicator. Thisthen enables us to read from the curvethe effect due to that applicator at thespecified distance, relative to that from apoint source at the same distance. Havingcalculated the effect of the point sourceat this distance, we have only to take thepercentage of it indicated by the readingon the Millimeters 3. Curve showing intensities of radiation requiredto produce erythema, for different filters, in terms ofintensity from unfiltered sources. Dotted curveshows portion near bend much enlarged. It should be noted that the intensityobtained is that at a point on the axis ofthe applicator. Except for a few specialcases, which will be mentioned later,intensities at other points have not beencalculated. The one given is evidentlythe maximum, since the average distanceto it from all points on the applicator isless than from any other. If it is a ques-tion of how much radiation can be appliedwithout causing injury, it is this maximumeffect which must be known. Let us consider some examples of theuse of the curves; 1. What is the effect 674 The Effect of the Size of Radium Applicators on Skin Doses clue to an applicator 4 cm. square, at adistance of 3 cm. from the skin, the filterbeing 2 mm. of
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