Röntgen rays are produced when the cathode rays fall upon, and as it were bombard, an obstacle of some kind. Almost any tube producing cathode rays will produce also Röntgen rays, but special forms of tube are used when the main object is to obtain these latter rays. Fig. 38 shows a typical form of simple X-ray tube. This, like all other tubes for X-ray work, is exhausted to a rather higher vacuum than tubes intended for the production of cathode rays only. The cathode C is made of aluminium, and is shaped like a saucer, its curvature being arranged so that the cathode rays are focused on to the anti-cathode A. The focusing as a rule is not done very accurately, for although sharper radiographs are obtained when the cathode rays converge exactly to a point on the anti-cathode, the heating effect at this point is so great that a hole is quickly burned. The target, or surface of the anti-cathode, is made of some metal having an extremely high melting-point, such as platinum, iridium, or tungsten. It has a flat surface inclined at an angle of about 45°, so that the rays emanating from it proceed in the direction shown by the dotted lines in the figure. The continuous lines show the direction of the cathode rays. The anode is made of aluminium, and it is shown at N. It is not necessary to have a separate anode, and the anti-cathode may be used as the anode. In the tube shown in Fig. 38 the anode and the anti-cathode are joined by an insulated wire, so that they both act as anodes. The tube is made of soda-glass, as the X-rays do not pass at all readily through lead-glass. [Illustration: _By permission of_] [_C. H. F. Muller._ FIG. 39.--Diagram of Mica Vacuum Regulator for X-Ray Tubes.]