THE DISCOVERY OF RONTGEN. 189 



pierce all materials in some way — organic tissue, flesh, wood, 

 paper, and so on ; therefore we suppose that the question here is 

 of longitudinal vibrations of the ether, whereas our ordinary light 

 consists of transverse vibrations. By chance, Rontgen observed 

 that the X beams made an impression on a photographic plate. 

 We may not think that the visible green phosphorescent light 

 produces this effect, for though the glass globe was enveloped in 

 a black opaque substance, so that no visible light pierced the 

 cover, an impression was nevertheless made on the sensitive plate. 

 The experiments which should prove all these miraculous qualities 

 of the X beams were quite recently made in the Polytechnic 

 School at Charlottenburg, and daylight was excluded from the 

 laboratory only while the sensitive photographic plate was being 

 put in the dark slide. Eight to twelve great accumulators of two 

 volts tension each were connected with a s-trong induction appara- 

 tus (Ruhmkorff), giving sparks of 30 cm. length. Copper wires 

 conducted the electric current to the electrodes of the glass globe, 

 which were kept joined to the mercurial air pump, to continue the 

 exhaustion of the air during the experiments. It seems the qual- 

 ity of the glass influences the success of the experiments, for of a 

 series of globes made by the glass-blower only two proved suitable. 

 The ordinary evacuated (exhausted) glass globes of commerce 

 were not suitable, probably because the vacuum was not sufficiently 

 complete. The right degree of evacuation of the air, the pressure 

 of which must not exceed a thousandth part of the atmospherical 

 pressure, is one of the necessary conditions. 



During the exhaustion of the glass globe a photographic plate 

 was put into the dark-slide in a dark-room. It is best to use a 

 dark-slide made of pasteboard, because the metallic parts of a 

 wooden one absorb the rays coming from the glass globe. The 

 object to be photographed was tied on the closed dark-slide by 

 silken threads, and placed with it in front of the glass tube. If we 

 wished to make an image of our hand, we placed the dark-slide 

 containing the plate in front of the glass globe at a distance of 

 25 cm., and put the hand on the pasteboard, no lens or photo- 

 graphic camera being used. An exposure of 25 minutes was 

 sufficient. If the distance is greater, the exposure must be longer, 

 as the X rays diminish with the quadrate (square) of the distance. 



