280 LIGHT AND ITS ARTIFICIAL PRODUCTION. 



Still more wonderful if I move a magnet along the tube. You can see 

 tbat the shadow of the cross moves with the motion of the magnet, 

 showing that the cathode rays are deflected by a magnet.' Only those 

 parts of the tube fluoresce to which I thus direct the cathode rays. If 

 I introduce into a Hittorf tube in the neighborhood of the cathode a bar 

 magnet, the cathode rays begin to rotate about the pole of the magnet 

 and the fluorescence glides in a circle around the magnet along the 

 sides of the tube. [Experiment.] In a similar manner it has been found 

 by difficult experiments that cathode rays can be reflected, etc. But as 

 long as they were confined it was difficult to get at them. Great prog- 

 ress was made when Lenard discovered a method of enticing the cath- 

 ode rays out of the tube. Hertz had found that the cathode rays pass 

 through aluminum with relative ease. Lenard constructed a Hittorf 

 tube with an opening at one end sealed air tight by a piece of sheet 

 aluminum one hundredth of a millimeter in thickness. On exhausting 

 this tube Lenard found that substances placed in front of the alumi- 

 num window could be made to fluoresce. In addition, these Lenard 

 rays showed almost the same peculiarities which made the X-rays, 

 discovered later, known to the whole civilized world. There was only 

 one fault to be found with them — their intensity was diminished by 

 passing through the aluminum and their action was therefore weak. 



How superior in this respect to the Lenard rays are the X-rays dis- 

 covered by W. 0. Eontgen. No window is needed for these. They 

 radiate freely from those portions of the tube on which the cathode 

 rays fall, into the exterior space, and pitilessly destroy photographic 

 plates which are placed as obstructions in their paths, however care- 

 fully they may be protected from the action of ordinary light. It is 

 remarkable that they should have remained undiscovered so long and 

 that they should have been discovered by happy accident. These 

 Eontgen rays after an existence of hardly a year have already been so 

 well known that I will confine myself to showing you the latest form 

 of X-ray tube of the firm of Siemens & Halske, which is so constructed 

 that the vacuum can be regulated at will. A screen coated with 

 barium platino- cyanide is so brightly illuminated as to be visible in the 

 farthest part of the hall. We now introduce between the X-ray tube 

 and the screen a wooden block 10 centimeters in thickness (4 inches). 

 On this is nailed a cross made of sheet lead, and you can probably all 

 see the shadow of this cross on the screen, while the wood, although 

 thirty times as thick, only gives the suggestion of a shadow. After 

 the lecture, however, you can individually convince yourselves of the 

 efficiency of these tubes. If the radiations are passed through a human 

 body the beating of the heart and the motion of the diaphragm in 

 breathing can be plainly observed. r- 



The Eontgen rays owe their immense popularity to their ability of 



1 Professor Goldstein lias discovered cathode rays which are not affected hy mag- 

 nets. 



