The theoretical investigations of the previous paper led to the result that in cross-beam treatment one must be guided by the following rules:—1. The crossing cones must envelop the regions to be treated as closely as feasible.2. There must be no redundancy of cones, which means that the diaphragms delimiting the cones ought to be co-ordinated.3. The layers of matter from the centre of the region of treatment to the surface must be equal for each port of entry and as small as possible.4. Compression ought to be applied simultaneously from all ports of entry. This compression, however, should not interface with a desirable displacement of organs out of the path of the beams.5. Whenever feasible a tetrahedron arrangement and, if possible, that of a regular tetrahedron is to be preferred. When this cannot be done for anatomical and physical reasons the arrangement of the regular triangle is the next best one.6. From the practical point of view we may add as a sixth rule that the arrangements, involving compression, addition of absorbing and scattering layers of matter and adjustments of beams in relation to the part to be treated must be obtainable with as little loss of time as possible and with a minimum of inconvenience to the patient, and be easily reproducible.