The mayneord contour projector (1) is a useful instrument in a variety of three-dimensional dosage problems. Such information as has been published concerning it deals only with symmetrical circular beams. A slight adaptation, which extends its application to rectangular fields and wedge filters, considerably increases its usefulness. Description Photographs of the apparatus are shown in Figure 1. I ts use with beams of circular cross section depends upon the fact that the isodose distribution is symmetrical about the central axis. Thus, distribution of radiation in any plane perpendicular to the axis will be in concentric circles, as shown in Figure 2. Doses in any plane other than the one through the axis might be determined if the intersection of this plane with the circles could be found. Intersections of such a plane, CCC'C', with three levels of isodose circles, are indicated in Figure 2. These points of intersection can be established with the contour projector. The movable steel pointer on the tri...