| Fig. 220. | Fig. 220-A. |
157. Cathode Rays. The cathode is the electrode of a vacuum-tube by which the current leaves the tube, and it has been known for some time that some kind of influence passes in straight lines from this point. Shadows, Fig. 219, are cast by such rays, a screen being placed in their path.
158. X-Rays. Professor Roentgen of Würzburg discovered that when the cathode rays are allowed to fall upon a solid body, the solid body gives out still other rays which differ somewhat from the original cathode rays. They can penetrate, more or less, through many bodies that are usually considered opaque. The hand, for example, may be used as a negative for producing a photograph of the bones, as the rays do not pass equally well through flesh and bone.
Fig. 221.
Fig. 220 shows a Crookes tube fitted with a metal plate, so that the cathode rays coming from C will strike it. The X-rays are given out from P. These rays are invisible and are even given out where the cathode rays strike the glass. Some chemical compounds are made luminous by these rays; so screens are made and coated with them in order that the shadows produced by the X-rays can be seen by the eye. Professor Roentgen named these the X-rays. Fig. 220-A shows a fluoroscope that contains a screen covered with proper chemicals.
Fig. 222.
