KATHODE AND RONTGEN RAYS 



of their course. When the Kathode rays fall upon 

 the glass walls of the tube they excite the glass to a 

 vivid, yellowish-green glow, but do not seem to escape 

 from the tube. 



In 1895 Rontgen found that when he brought 

 fluorescent substances, though enclosed in thick paste- 

 board cases, near the tube they would glow with a 

 phosphorent light, though no light from the tube 

 itself was apparent. These invisible rays seemed to 

 radiate from a certain part of the yellowish-green 

 lighted surface. They were not Kathode rays, for 

 they had not the characteristic property of being at- 

 tracted by the Magnet. For these new rays, that 

 Rontgen provisionally called X-rays (unknown rays), 

 all bodies are more or less transparent. They will 

 pass through a thick book of 1000 pages, through thick 

 blocks of wood, and even through metallic plates, if 

 not too thick. The permeability of plates seems some- 

 what dependent inversely upon their specific gravity. 

 A plate of lead ^ inches thick is nearly impenetrable, 

 while one of aluminum ten times as thick is penetrated. 

 The X-rays are neither reflected nor refracted. The 

 Rontgen rays, invisible themselves, cast the shadows 

 of difficultly, penetrable bodies permanently upon the 

 photographic plate, or, if falling upon a fluorescent 

 substance, produce visible shadows thereon of the 

 dense body that cut off their rays, the said rays pass- 



189 



