THE purpose of the investigation was to determine the class of work in which the Philips type of Muller Metalix tubes will produce better roentgenograms than the Coolidge tubes. Apparatus For the investigation, two Muller Metalix tubes of the Philips type and five Coolidge tubes were used, as listed below, the focal spot sizes being measured by the pinhole method. 1. 3 K.W. Metalix Radiator, focal spot 3.5 × 3.5 mm. 2. 6 K.W. Metalix Water-cooled, focal spot 3.5 × 3.75 mm. 3. 10 milliamperes Coolidge, focal spot 3 × 3 mm. 4. 10 milliamperes Coolidge, focal spot 4 × 4 mm. 5. 30 milliamperes Coolidge, focal spot 4.75 × 5 mm. 6. 100 milliamperes Coolidge, focal spot 8 × 8.5 mm. 7. B. F. Universal, focal spot 7.5 × 11 mm. The energy for the tubes was supplied by a double cross arm rectifying switch type of machine, with all exposure factors controlled and carefully checked. Observation of Detail in Roentgenograms To analyze the detail of structure, wire mesh was used. The distances that the mesh was ra...