such that the diaphragmed pyramidal beam configuration 25 will have its vertex lying at the focus 26 of the x ray tube 27 each close to focus diaphragm plate 16 through 19 fig2 and the respective parallel remote from focus diaphragm plate 20 through 23 oppositely positioned in relation to the longitudinal axis 24 of the diaphragm assembly are mounted on levers which are pivotably mounted about an axis lying between such diaphragm plates and parallel thereto said axis being common to all four parallel diaphragm plates for each set of four parallel diaphragm plates two levers are provided which are secured to the opposite ends of the diaphragm plates as shown in fig2 thus the set of four diaphragm plates 16 21 and 17 20 fig2 are connected to one another by means of levers 28 29 and 30 31 and are swingable in a scissors like fashion about the lever pivot arbors 32 33 fig2 which lie on a common axis 32a the outer ends of arbors 32 33 are rigidly connected to supports 5 and 7 fig2 the four additional diaphragm plates 18 23 and 19 22 fig2 which are staggered 90 u00b0 relative to diaphragm plates 16 17 20 21 are connected to one another by means of levers 34 35.