xi ACROSS THE BORDER 303 



window, was able to pass the rays out into the air, 

 without reducing their properties of producing phos- 

 phorescence. He further showed that the rays thus 

 set free from their bondage in a vacuum were capable 

 of recording their existence upon a photographic plate. 

 Evidently all that was necessary to be done in order to 

 utilise this knowledge was to elaborate the experiments 

 by placing various substances in the path of the rays 

 and catching their shadows upon a sensitised photo- 

 graphic plate. This is what Prof. Rontgen did, but he 

 found also that the aluminium window was unnecessary ; 

 for the rays escape through glass. 



A vacuum-tube rendered luminous by the electric 

 discharge, was surrounded by Prof. Rontgen with a 

 shield of black paper in a completely darkened room. 

 A piece of paper, having one side covered with a phos- 

 phorescent substance, was brought into the neighbour- 

 hood of the tube, and found to become brilliantly 

 luminous, in spite of the fact that the light of the vacuum- 

 tube was covered up. This proved beyond doubt that 

 certain rays can pass through paper, and still possess 

 the ability to produce fluorescence. A book of 1,000 

 pages did not prevent this action, nor did two packs 

 of cards, thick blocks of wood, or ebonite, while a 

 sheet of aluminium nearly an inch thick only reduced the 

 effect. Plates of copper, silver, lead, gold and platinum 

 permitted the rays to pass, but only when they were 

 thin. When the hand was held between the vacuum- 

 tube and the fluorescent screen, the shadow showed 

 the bones darkly, with only faint outlines of the sur- 

 rounding flesh, the reason being that the bones 

 are almost opaque to the active rays, while the flesh 

 is transparent to them. By substituting a sensitive 

 plate for the fluorescent screen, a photograph could, 



