ELECTRICITY AND MAGNETISM 



patently arising from them, and are what have since 

 been called the Roentgen rays, or X-rays. 



Roentgen found that a shadow is thrown upon the 

 screen by substances held between it and the exhausted 

 tube, the character of the shadow depending upon the 

 density of the substance. Thus metals are almost 

 completely opaque to the rays; such substances as 

 bone much less so, and ordinary flesh hardly so at all. 

 If a coin were held in the hand that had been inter- 

 posed between the tube and the screen the picture 

 formed showed the coin as a black shadow; and the 

 bones of the hand, while casting a distinct shadow, 

 showed distinctly lighter; while the soft tissues pro- 

 duced scarcely any shadow at all. The value of such 

 a discovery was obvious from the first; and was still 

 further enhanced by the discovery made shortly that 

 photographic plates are affected by the rays, thus 

 making it possible to make permanent photographic 

 records of pictures through what we know as opaque 

 substances. 



\Yhat adds materially to the practical value of 

 Roentgen's discovery is the fact that the apparatus for 

 producing the X-rays is now so simple and relatively 

 inexpensive that it is within the reach even of amateur 

 scientists. It consists essentially of an induction coil 

 attached either to cells or a street-current plug for gen- 

 erating the electricity, a focus tube, and a phospho- 

 rescence screen. These focus tubes are made in various 

 shapes, but perhaps the most popular are in the form 

 of a glass globe, not unlike an ordinary small-sized 

 water-bottle, this tube being closed and exhausted, 

 and having the two poles (anode and cathode) sealed 



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