THE BECQUEREL RAYS 137 



aluminium half an inch thick, or a wooden board, lumin- 

 escence was still produced. Rontgen investigated the 

 transparency of various objects, and soon discovered that 

 while skin and flesh are nearly transparent to these 

 radiations, bone is comparatively opaque, and may be 

 made to throw its shadow on a photographic plate or on a 

 screen covered by phosphorescent material. The surgical 

 bearing of this discovery was at once evident ; and by help 

 of ' skiographs ' or shadow-writing the presence of a bullet 

 embedded in the tissue can be recognised, and its exact 

 situation localised ; and in cases of fractures of bones, 

 their exact shape can be made out, and they can be 

 successfully set, for it is always possible to examine the 

 position of the fractured ends through envelopes of 

 bandages, which themselves are nearly transparent to 

 Rontgen or X-rays. 



One of the most remarkable properties of these rays is 

 that they cannot be refracted by passage through a prism, 

 nor apparently reflected from any object, however smooth 

 and well-polished, nor can they be polarised. They are, 

 however, absorbed by different substances unequally, and 

 apparently the denser the substance the greater its absorb- 

 ing power. 



It might be supposed at first blush that the X-rays of 

 Rontgen were identical with cathode rays. But if this 

 were the case the X-rays should pass straight from the 

 cathode through the walls of the tube, and proceed in a 

 straight line ; as a matter of fact, their point of origin can 

 be displaced with a magnet, and if a spherical bulb be 

 used to contain the cathode, each point on the bulb is a 

 centre of emission, sending its radiations in all directions. 

 Now Lenard had recognised that cathode rays could be 

 differentiated into two distinct kinds. Suppose that they 

 were made to pass through a hole in a block of lead, and to 

 impinge on a photographic plate, if a magnet were placed 



